[This was originally posted in the Unity forums]

Since I did not find any built-in functionality to make shadow casters fit in the shape of tilemaps (something that I think is fundamental) I made my own script to achieve such thing without having to manually create and adjust every shadow caster.

Just add the script to your project and you will see a new option in the top menu of the editor (Tools–>Generate Shadow Casters). Once you execute it, shadow casters will be created for all the tilemaps in the open scene. It requires that the tilemaps have CompositeCollider2Ds. I made it as generic as possible so it is not limited to tilemaps, it only depends on composite colliders.

I hope it helps you to save some time.

// Copyright 2020 Alejandro Villalba Avila
//
// Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"),
// to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense,
// and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
// IN THE SOFTWARE.
 
using System.Collections.Generic;
using System.Reflection;
using UnityEngine;
using UnityEngine.Experimental.Rendering.Universal;
 
/// <summary>
/// It extends the ShadowCaster2D class in order to be able to modify some private data members.
/// </summary>
public static class ShadowCaster2DExtensions
{
    /// <summary>
    /// Replaces the path that defines the shape of the shadow caster.
    /// </summary>
    /// <remarks>
    /// Calling this method will change the shape but not the mesh of the shadow caster. Call SetPathHash afterwards.
    /// </remarks>
    /// <param name="shadowCaster">The object to modify.</param>
    /// <param name="path">The new path to define the shape of the shadow caster.</param>
    public static void SetPath(this ShadowCaster2D shadowCaster, Vector3[] path)
    {
        FieldInfo shapeField = typeof(ShadowCaster2D).GetField("m_ShapePath",
                                                               BindingFlags.NonPublic |
                                                               BindingFlags.Instance);
        shapeField.SetValue(shadowCaster, path);
    }
 
    /// <summary>
    /// Replaces the hash key of the shadow caster, which produces an internal data rebuild.
    /// </summary>
    /// <remarks>
    /// A change in the shape of the shadow caster will not block the light, it has to be rebuilt using this function.
    /// </remarks>
    /// <param name="shadowCaster">The object to modify.</param>
    /// <param name="hash">The new hash key to store. It must be different from the previous key to produce the rebuild. You can use a random number.</param>
    public static void SetPathHash(this ShadowCaster2D shadowCaster, int hash)
    {
        FieldInfo hashField = typeof(ShadowCaster2D).GetField("m_ShapePathHash",
                                                              BindingFlags.NonPublic |
                                                              BindingFlags.Instance);
        hashField.SetValue(shadowCaster, hash);
    }
}
 
/// <summary>
/// It provides a way to automatically generate shadow casters that cover the shapes of composite colliders.
/// </summary>
/// <remarks>
/// Specially recommended for tilemaps, as there is no built-in tool that does this job at the moment.
/// </remarks>
public class ShadowCaster2DGenerator
{
 
#if UNITY_EDITOR
 
    [UnityEditor.MenuItem("Generate Shadow Casters", menuItem = "Tools/Generate Shadow Casters")]
    public static void GenerateShadowCasters()
    {
        CompositeCollider2D[] colliders = GameObject.FindObjectsOfType<CompositeCollider2D>();
 
        for(int i = 0; i < colliders.Length; ++i)
        {
            GenerateTilemapShadowCastersInEditor(colliders[i], false);
        }
    }
 
    [UnityEditor.MenuItem("Generate Shadow Casters (Self Shadows)", menuItem = "Tools/Generate Shadow Casters (Self Shadows)")]
    public static void GenerateShadowCastersSelfShadows()
    {
        CompositeCollider2D[] colliders = GameObject.FindObjectsOfType<CompositeCollider2D>();
 
        for (int i = 0; i < colliders.Length; ++i)
        {
            GenerateTilemapShadowCastersInEditor(colliders[i], true);
        }
    }
 
    /// <summary>
    /// Given a Composite Collider 2D, it replaces existing Shadow Caster 2Ds (children) with new Shadow Caster 2D objects whose
    /// shapes coincide with the paths of the collider.
    /// </summary>
    /// <remarks>
    /// It is recommended that the object that contains the collider component has a Composite Shadow Caster 2D too.
    /// It is recommended to call this method in editor only.
    /// </remarks>
    /// <param name="collider">The collider which will be the parent of the new shadow casters.</param>
    /// <param name="selfShadows">Whether the shadow casters will have the Self Shadows option enabled..</param>
    public static void GenerateTilemapShadowCastersInEditor(CompositeCollider2D collider, bool selfShadows)
    {
        GenerateTilemapShadowCasters(collider, selfShadows);
 
        UnityEditor.SceneManagement.EditorSceneManager.MarkAllScenesDirty();
    }
 
#endif
 
    /// <summary>
    /// Given a Composite Collider 2D, it replaces existing Shadow Caster 2Ds (children) with new Shadow Caster 2D objects whose
    /// shapes coincide with the paths of the collider.
    /// </summary>
    /// <remarks>
    /// It is recommended that the object that contains the collider component has a Composite Shadow Caster 2D too.
    /// It is recommended to call this method in editor only.
    /// </remarks>
    /// <param name="collider">The collider which will be the parent of the new shadow casters.</param>
    /// <param name="selfShadows">Whether the shadow casters will have the Self Shadows option enabled..</param>
    public static void GenerateTilemapShadowCasters(CompositeCollider2D collider, bool selfShadows)
    {
        // First, it destroys the existing shadow casters
        ShadowCaster2D[] existingShadowCasters = collider.GetComponentsInChildren<ShadowCaster2D>();
 
        for (int i = 0; i < existingShadowCasters.Length; ++i)
        {
            if(existingShadowCasters[i].transform.parent != collider.transform)
            {
                continue;
            }
 
            GameObject.DestroyImmediate(existingShadowCasters[i].gameObject);
        }
 
        // Then it creates the new shadow casters, based on the paths of the composite collider
        int pathCount = collider.pathCount;
        List<Vector2> pointsInPath = new List<Vector2>();
        List<Vector3> pointsInPath3D = new List<Vector3>();
 
        for (int i = 0; i < pathCount; ++i)
        {
            collider.GetPath(i, pointsInPath);
 
            GameObject newShadowCaster = new GameObject("ShadowCaster2D");
            newShadowCaster.isStatic = true;
            newShadowCaster.transform.SetParent(collider.transform, false);
 
            for(int j = 0; j < pointsInPath.Count; ++j)
            {
                pointsInPath3D.Add(pointsInPath[j]);
            }
 
            ShadowCaster2D component = newShadowCaster.AddComponent<ShadowCaster2D>();
            component.SetPath(pointsInPath3D.ToArray());
            component.SetPathHash(Random.Range(int.MinValue, int.MaxValue)); // The hashing function GetShapePathHash could be copied from the LightUtility class
            component.selfShadows = selfShadows;
            component.Update();
 
            pointsInPath.Clear();
            pointsInPath3D.Clear();
        }
    }
}

If you want to generate it in runtime, on Awake, for a specific object:

    // Copyright 2021 Alejandro Villalba Avila
    //
    // Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"),
    // to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense,
    // and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
    //
    // The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
    //
    // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
    // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
    // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
    // IN THE SOFTWARE.
     
    using UnityEngine;
    using UnityEngine.Experimental.Rendering.Universal;
     
    namespace Game.Utils
    {
        /// <summary>
        /// Generates ShadowCaster2Ds for every continuous block of a tilemap on Awake, applying some settings.
        /// </summary>
        public class TilemapShadowCaster2D : MonoBehaviour
        {
            [SerializeField]
            protected CompositeCollider2D m_TilemapCollider;
     
            [SerializeField]
            protected bool m_SelfShadows = true;
     
            protected virtual void Reset()
            {
                m_TilemapCollider = GetComponent<CompositeCollider2D>();
            }
     
            protected virtual void Awake()
            {
                ShadowCaster2DGenerator.GenerateTilemapShadowCasters(m_TilemapCollider, m_SelfShadows);
            }
        }
    }

Known issues

Beware of generating shadow casters for “closed” tilemaps. The problem is that the closed shapes that the composite collider generates, for example, in a ring-shaped tilemap, are 2 overlapping convex meshes that enclose the point light. When the renderer processes the geometry to obtain the shadowed areas it produces undesired results. The following images are taken using RenderDoc, they show what the GPU writes to intermediary textures for a point light. The black area is the part where the light passes the stencil test.

A workaround: Remove one tile from the walls of the tilemap so it is not closed, and fill that hole putting a tile in another layer of the tilemap.

Potential improvements

One of the devs from Unity, MolvMay, suggested that they added a way to read the actual primitive shapes/geometry produced from any Collider2D in a super efficient way in version 2021.2, which may hellp to improve performance.