Simple Fog (URP)

fog.shader

Shader "Hidden/FogShader"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
    }
    SubShader
    {
        // No culling or depth
        Cull Off ZWrite Off ZTest Always

        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag

            #include "UnityCG.cginc"

            struct appdata
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;
            };

            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
            };

            v2f vert (appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = v.uv;
                return o;
            }

            sampler2D _MainTex;
            sampler _CameraDepthTexture;
            float3 _FogColor;

            fixed4 frag (v2f i) : SV_Target
            {
                float4 depthColor = tex2D(_CameraDepthTexture, i.uv);
                float linearDepth = Linear01Depth(depthColor.r);
                float fogDensity = linearDepth;
                
                float4 mainTexColor = tex2D(_MainTex, i.uv);
                float4 outColor = mainTexColor;
                outColor.rgb = lerp(mainTexColor.rgb, _FogColor, fogDensity);
                return outColor;
            }
            ENDCG
        }
    }
}

TestRendererFeature.cs

using UnityEngine;
using UnityEngine.Rendering;
using UnityEngine.Rendering.Universal;

public class TestRendererFeature : ScriptableRendererFeature
{
    private TestRenderPass testRenderPass;
    public Material fogMaterial;
    public Color fogColor;

    class TestRenderPass : ScriptableRenderPass
    {
        private Material mat;
        private Color color;
        private RenderTargetIdentifier source;

        private readonly int temporaryRTId = Shader.PropertyToID("_TempRT");

        public TestRenderPass(Material _mat, Color _color) {
            mat = _mat;
            color = _color;
        }

        public override void OnCameraSetup(CommandBuffer cmd, ref RenderingData renderingData) {
            source = renderingData.cameraData.renderer.cameraColorTarget;
        }

        public override void Execute(ScriptableRenderContext _context, ref RenderingData _renderingData) {
            CommandBuffer cmd = CommandBufferPool.Get(name: "TestRenderPass");

            RenderTextureDescriptor descriptor = _renderingData.cameraData.cameraTargetDescriptor;
            descriptor.depthBufferBits = 0;

            cmd.GetTemporaryRT(temporaryRTId, descriptor, FilterMode.Bilinear);
            RenderTargetIdentifier rtIdentifier = new RenderTargetIdentifier(temporaryRTId);

            cmd.CopyTexture(source, rtIdentifier);

            mat.SetColor("_FogColor", color);
            cmd.Blit(rtIdentifier, source, mat);

            cmd.ReleaseTemporaryRT(temporaryRTId);

            _context.ExecuteCommandBuffer(cmd);
            CommandBufferPool.Release(cmd);
        }
    }

    public override void Create() {
        testRenderPass = new TestRenderPass(fogMaterial, fogColor);
        testRenderPass.renderPassEvent = RenderPassEvent.AfterRenderingSkybox;
    }

    public override void AddRenderPasses(ScriptableRenderer renderer, ref RenderingData renderingData) {
        if (fogMaterial != null) {
            renderer.EnqueuePass(testRenderPass);
        }
    }
}