Simple Fog Shader

FogComponent.cs

using UnityEngine;

[RequireComponent(typeof(Camera))]
public class FogComponent : MonoBehaviour
{
    public Shader myFogShader;
    public Color fogColor = Color.red;
    
    private Material myFogMaterial;

    // Start is called before the first frame update
    void Start()
    {
        GetComponent<Camera>().depthTextureMode = DepthTextureMode.Depth;
        myFogMaterial = new Material(myFogShader);
        myFogMaterial.hideFlags = HideFlags.HideAndDontSave;
    }

    private void OnRenderImage(RenderTexture src, RenderTexture dest) {
        myFogMaterial.SetColor("_FogColor", fogColor);
        Graphics.Blit(src, dest, myFogMaterial);
    }
}

FogShader.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
        }
    }
}