The transparent shader receiving the shadow.

Shader "FX/Matte Shadow" {
 
Properties {
    _Color ("Main Color", Color) = (1,1,1,1)
    _MainTex ("Base (RGB) Trans (A)", 2D) = "white" {}
    _Cutoff ("Alpha cutoff", Range(0,1)) = 0.5
}
 
SubShader {
 
    Tags {"Queue"="AlphaTest" "IgnoreProjector"="True" "RenderType"="TransparentCutout"}
    LOD 200
    Blend Zero SrcColor
 
CGPROGRAM
 
#pragma surface surf ShadowOnly alphatest:_Cutoff
 
fixed4 _Color;
 
struct Input {
    float2 uv_MainTex;
};
 
inline fixed4 LightingShadowOnly (SurfaceOutput s, fixed3 lightDir, fixed atten)
{
    fixed4 c;
    c.rgb = s.Albedo*atten;
    c.a = s.Alpha;
 
    return c;
}
 
void surf (Input IN, inout SurfaceOutput o) 
{
 
    fixed4 c = _Color; 
    o.Albedo = c.rgb;
    o.Alpha = 1;
 
}
 
ENDCG
 
}
 
Fallback "Transparent/Cutout/VertexLit"
 

}

Transparent shader that accepts the shadow

Shader "GreenArch/Transparent Plane Shadow2" {
    Properties {
    }
    SubShader {
        Tags {
            "RenderType"="Opaque"
        }
        Pass {
            Name "DEFERRED"
            Tags {
                "LightMode"="Deferred"
            }
            
            
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #define UNITY_PASS_DEFERRED
            #include "UnityCG.cginc"
            #include "UnityPBSLighting.cginc"
            #include "UnityStandardBRDF.cginc"
            #pragma fragmentoption ARB_precision_hint_fastest
            #pragma multi_compile_shadowcaster
            #pragma multi_compile ___ UNITY_HDR_ON
            #pragma multi_compile_fog
            #pragma exclude_renderers gles3 metal d3d11_9x xbox360 xboxone ps3 ps4 psp2 
            #pragma target 3.0
            struct VertexInput {
                float4 vertex : POSITION;
                float3 normal : NORMAL;
            };
            struct VertexOutput {
                float4 pos : SV_POSITION;
                float4 posWorld : TEXCOORD0;
                float3 normalDir : TEXCOORD1;
            };
            VertexOutput vert (VertexInput v) {
                VertexOutput o = (VertexOutput)0;
                o.normalDir = UnityObjectToWorldNormal(v.normal);
                o.posWorld = mul(unity_ObjectToWorld, v.vertex);
                o.pos = mul(UNITY_MATRIX_MVP, v.vertex );
                return o;
            }
            void frag(
                VertexOutput i,
                out half4 outDiffuse : SV_Target0,
                out half4 outSpecSmoothness : SV_Target1,
                out half4 outNormal : SV_Target2,
                out half4 outEmission : SV_Target3 )
            {
                i.normalDir = normalize(i.normalDir);
                float3 viewDirection = normalize(_WorldSpaceCameraPos.xyz - i.posWorld.xyz);
                float3 normalDirection = i.normalDir;
                float3 viewReflectDirection = reflect( -viewDirection, normalDirection );
////// Lighting:
                float3 finalColor = 0;
                outDiffuse = half4( 0, 0, 0, 1 );
                outSpecSmoothness = half4(0,0,0,0);
                outNormal = half4( normalDirection * 0.5 + 0.5, 1 );
                outEmission = half4(0,0,0,1);
                #ifndef UNITY_HDR_ON
                    outEmission.rgb = exp2(-outEmission.rgb);
                #endif
            }
            ENDCG
        }
        Pass {
            Name "FORWARD"
            Tags {
                "LightMode"="ForwardBase"
            }
            
            
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #define UNITY_PASS_FORWARDBASE
            #include "UnityCG.cginc"
            #include "UnityPBSLighting.cginc"
            #include "UnityStandardBRDF.cginc"
            #pragma multi_compile_fwdbase_fullshadows
            #pragma multi_compile_fog
            #pragma exclude_renderers gles3 metal d3d11_9x xbox360 xboxone ps3 ps4 psp2 
            #pragma target 3.0
            struct VertexInput {
                float4 vertex : POSITION;
                float3 normal : NORMAL;
            };
            struct VertexOutput {
                float4 pos : SV_POSITION;
                float4 posWorld : TEXCOORD0;
                float3 normalDir : TEXCOORD1;
                UNITY_FOG_COORDS(2)
            };
            VertexOutput vert (VertexInput v) {
                VertexOutput o = (VertexOutput)0;
                o.normalDir = UnityObjectToWorldNormal(v.normal);
                o.posWorld = mul(unity_ObjectToWorld, v.vertex);
                o.pos = mul(UNITY_MATRIX_MVP, v.vertex );
                UNITY_TRANSFER_FOG(o,o.pos);
                return o;
            }
            float4 frag(VertexOutput i) : COLOR {
                i.normalDir = normalize(i.normalDir);
                float3 viewDirection = normalize(_WorldSpaceCameraPos.xyz - i.posWorld.xyz);
                float3 normalDirection = i.normalDir;
                float3 viewReflectDirection = reflect( -viewDirection, normalDirection );
////// Lighting:
                float3 finalColor = 0;
                fixed4 finalRGBA = fixed4(finalColor,1);
                UNITY_APPLY_FOG(i.fogCoord, finalRGBA);
                return finalRGBA;
            }
            ENDCG
        }
    }
    FallBack "Diffuse"
    CustomEditor "ShaderForgeMaterialInspector"
}

//This is based on a shader from https://alastaira.wordpress.com/2014/12/30/adding-shadows-to-a-unity-vertexfragment-shader-in-7-easy-steps/

Shader "Custom/MobileARShadow"
{
    SubShader {
        Pass {
         
            // 1.) This will be the base forward rendering pass in which ambient, vertex, and
            // main directional light will be applied. Additional lights will need additional passes
            // using the "ForwardAdd" lightmode.
            // see: http://docs.unity3d.com/Manual/SL-PassTags.html
            Tags { "LightMode" = "ForwardBase" "RenderType"="Opaque" "Queue"="Geometry+1" "ForceNoShadowCasting"="True"  }
			LOD 150
			Blend Zero SrcColor
			ZWrite On
        
            CGPROGRAM
 
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"
 
            // 2.) This matches the "forward base" of the LightMode tag to ensure the shader compiles
            // properly for the forward bass pass. As with the LightMode tag, for any additional lights
            // this would be changed from _fwdbase to _fwdadd.
            #pragma multi_compile_fwdbase
 
            // 3.) Reference the Unity library that includes all the lighting shadow macros
            #include "AutoLight.cginc"
 
 
            struct v2f
            {
                float4 pos : SV_POSITION;
                 
                // 4.) The LIGHTING_COORDS macro (defined in AutoLight.cginc) defines the parameters needed to sample 
                // the shadow map. The (0,1) specifies which unused TEXCOORD semantics to hold the sampled values - 
                // As I'm not using any texcoords in this shader, I can use TEXCOORD0 and TEXCOORD1 for the shadow 
                // sampling. If I was already using TEXCOORD for UV coordinates, say, I could specify
                // LIGHTING_COORDS(1,2) instead to use TEXCOORD1 and TEXCOORD2.
                LIGHTING_COORDS(0,1)
            };
 
 
            v2f vert(appdata_base v) {
                v2f o;
                o.pos = UnityObjectToClipPos (v.vertex);
                 
                // 5.) The TRANSFER_VERTEX_TO_FRAGMENT macro populates the chosen LIGHTING_COORDS in the v2f structure
                // with appropriate values to sample from the shadow/lighting map
                TRANSFER_VERTEX_TO_FRAGMENT(o);
                 
                return o;
            }
 
            fixed4 frag(v2f i) : COLOR {
             
                // 6.) The LIGHT_ATTENUATION samples the shadowmap (using the coordinates calculated by TRANSFER_VERTEX_TO_FRAGMENT
                // and stored in the structure defined by LIGHTING_COORDS), and returns the value as a float.
                float attenuation = LIGHT_ATTENUATION(i);
                return fixed4(1.0,1.0,1.0,1.0) * attenuation;
            }
 
            ENDCG
        }
    }
     
    // 7.) To receive or cast a shadow, shaders must implement the appropriate "Shadow Collector" or "Shadow Caster" pass.
    // Although we haven't explicitly done so in this shader, if these passes are missing they will be read from a fallback
    // shader instead, so specify one here to import the collector/caster passes used in that fallback.
    Fallback "VertexLit"

}