Make objects transparent without affecting the shadow?

Edit: Below is shader to do what you want.. but I don't think you actually need it!!! In the MeshRenderer component, under lighting, there is a setting under "cast shadow" called "Shadows only", which does exactly what you're looking for.. and I think it will work with any material. The shader version does have one advantage though: rather than being just on or off, you can fade out the material's color's alpha slowly, without affecting the shadow.

Shader:

Here is a simple vertex shader that does that. Note that the second pass, is where the shadow is drawn.

• The material/shader has a parameter called hide. Set this value to Zero to display the object & show. Set it to not-zero to hide the object but not the shadow: (this flag simply replaces the _Color alpha with 0)

• Or, you can simply adjust the material's color's alpha manually.

  ---

(I probably have more #includes and #pragama than necessary, but didn't want to experiment to see which ones could be removed.)

 Shader "Unlit/InvisWithShadow"
 {
     Properties
     {
         _Color("Main Color", Color) = (1,1,1,1)
         _MainTex("Texture", 2D) = "white" {}
         _Hide("Hide", Float) = 0.1
     }
 
     SubShader
     {
         Tags
         {
             "RenderType" = "Transparent"
             "Queue" = "Transparent"
         }
         Blend SrcAlpha OneMinusSrcAlpha
         Pass
         {
             Name "MAPHEIGHT"
             Tags{ "LightMode" = "ForwardBase" }
             ZWrite On
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             #pragma multi_compile_fog    
             #include "UnityCG.cginc"
             #include "Lighting.cginc"
             #pragma multi_compile_fwdbase nolightmap nodirlightmap nodynlightmap novertexlight
             #include "AutoLight.cginc"            
 
             float4 _Color;
             float _Hide;
             //sampler2D _MainTex;
 
             struct v2f {
                 half2 uv : TEXCOORD3;
                 float4 pos : SV_POSITION;
             };
 
 
             v2f vert(float4 vertex : POSITION, float3 normal : NORMAL,float2 uv : TEXCOORD0)
             {
                 v2f o;
                 o.pos = UnityObjectToClipPos(vertex);
                 o.uv = uv;
                 TRANSFER_VERTEX_TO_FRAGMENT(o);
                 TRANSFER_SHADOW(o);
                 return o;
             }
 
             sampler2D _MainTex;
             fixed4 frag(v2f i) : SV_Target
             {
                 fixed4 c = tex2D(_MainTex, i.uv)* _Color;
                 if(_Hide)
                     c.a = 0;
                 return c;
             }
 
             ENDCG
         }
             // shadow casting support
             // shadow caster rendering pass, implemented manually
         Pass
         {
             Name "SHADOWCASTER"
             Tags{ "LightMode" = "ShadowCaster" }
 
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             #pragma multi_compile_shadowcaster
             #include "UnityCG.cginc"
 
             struct v2f {
                 V2F_SHADOW_CASTER;
             };
 
             v2f vert(appdata_base v)
             {
                 v2f o;
                 TRANSFER_SHADOW_CASTER_NORMALOFFSET(o)
                     return o;
             }
 
             float4 frag(v2f i) : SV_Target
             {
                 SHADOW_CASTER_FRAGMENT(i)
             }
             ENDCG
         }
     }//end subshader
 }//end shader