DepthGrayscale.shader

// via http://williamchyr.com/2013/11/unity-shaders-depth-and-normal-textures/

Shader "Custom/DepthGrayscale" {
	SubShader{
		Tags{ "RenderType" = "Opaque" }

		Pass{
		CGPROGRAM
#pragma vertex vert
#pragma fragment frag
#include "UnityCG.cginc"

		sampler2D _CameraDepthTexture;

	struct v2f
	{
		float4 pos : SV_POSITION;
		float4 scrPos:TEXCOORD1;
	};

	//Vertex Shader
	v2f vert(appdata_base v)
	{
		v2f o;
		o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
		o.scrPos = ComputeScreenPos(o.pos);
		//for some reason, the y position of the depth texture comes out inverted
		//o.scrPos.y = 1 - o.scrPos.y;
		return o;
	}

	//Fragment Shader
	half4 frag(v2f i) : COLOR{
		float depthValue = Linear01Depth(tex2Dproj(_CameraDepthTexture, UNITY_PROJ_COORD(i.scrPos)).r);
	half4 depth;

	depth.r = depthValue;
	depth.g =  depthValue;
	depth.b = depthValue;

	depth.a = 1;
	return depth;
	}
		ENDCG
	}
	}
		FallBack "Diffuse"
}

RenderTextureTest.cs

using UnityEngine;
using System.Collections;

// ATTACHED ON THE CAMERA

[ExecuteInEditMode]
public class RenderTextureTest : MonoBehaviour
{

    // https://docs.unity3d.com/ScriptReference/Texture2D.ReadPixels.html

    public Material mat;
    public Texture2D tex;

    public RenderTexture ShadowRenderTexture;

    private Camera _camera;

    void Start()
    {
        _camera = GetComponent<Camera>();
        _camera.depthTextureMode = DepthTextureMode.Depth;

        ShadowRenderTexture = _camera.targetTexture;

    }


    void OnRenderImage(RenderTexture source, RenderTexture destination)
    {
        Graphics.Blit(source, destination, mat);

        //mat is the material which contains the shader
        //we are passing the destination RenderTexture to
    }
}

works in URP ?