参考地址：http://www.manew.com/thread-110080-1-1.html
总结：
获取顶点的世界坐标，根据世界坐标的y计算出雾化的比例，和最终颜色进行差值。

Shader "Custom/Fog_cg"
{
    Properties
    {
        _Color("Color",Color)=(1,1,1,1)
        _MainTex("Texture",2D) = "white"{}
        _FogColor("Fog Color",Color) = (0.3,0.4,0.7,1.0)
        _FogStart("Fog Start Y", float) = 0
        _FogEnd("Fog End Y",float) = 0
    }
    SubShader
    {
        Tags { "RenderType"="Opaque" }

        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;
                half fog  :TEXCOORD1;
            };

            fixed4 _Color;

            fixed4 _FogColor;

            half _FogStart;

            half _FogEnd;

            sampler2D _MainTex;
            float4 _MainTex_ST;
            
            v2f vert (appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = TRANSFORM_TEX(v.uv, _MainTex);

                float4 pos = mul(unity_ObjectToWorld, v.vertex).xyzw;
                o.fog = saturate((_FogStart - pos.y) / (_FogStart - _FogEnd));
                return o;
            }

            fixed4 frag(v2f i) : SV_Target
            {
                // sample the texture
                fixed4 col = tex2D(_MainTex, i.uv);
                fixed3 fogColor = _FogColor.rgb;
                fixed3 tintColor = _Color.rgb;
                col.rgb = lerp(col.rgb*tintColor, fogColor, i.fog);
                return col;
            }
            ENDCG
        }
    }
}

效果：

image.png