A shader that discards the part of a mesh that's on one side of a plane.

Spatial Cutoff Test.shader

// Upgrade NOTE: replaced 'PositionFog()' with multiply of UNITY_MATRIX_MVP by position
// Upgrade NOTE: replaced 'V2F_POS_FOG' with 'float4 pos : SV_POSITION'

// Upgrade NOTE: replaced 'glstate.matrix.projection' with 'UNITY_MATRIX_P'

Shader "Mine/Spatial Cutoff Test"
{
    Properties
    {
        _Color ("Main Color", Color) = (1,1,1,1)
        _MainTex ("Base (RGB)", 2D) = "white" {}
        _Cutoff ("Cutoff", Float) = 1
    }

    SubShader
    {
        Pass
        {

CGPROGRAM
// Upgrade NOTE: excluded shader from Xbox360; has structs without semantics (struct v2f members newPos)
#pragma exclude_renderers xbox360
#pragma vertex vert
#pragma fragment frag
#pragma fragmentoption ARB_fog_exp2
#include "UnityCG.cginc"

float4 _Color;
sampler2D _MainTex;
float _Cutoff;

struct v2f
{
    float4 pos : SV_POSITION;
    float2 uv : TEXCOORD0;
    float4 newPos;
};

v2f vert(appdata_base v)
{
    v2f o;
    o.pos = mul (UNITY_MATRIX_MVP, v.vertex);
    o.uv = TRANSFORM_UV(0);
    o.newPos = mul(UNITY_MATRIX_P, v.vertex);
    return o;
}

half4 frag(v2f i) : COLOR
{
    half4 texcol = tex2D(_MainTex, i.uv);

    if(i.newPos[2] > _Cutoff)
        discard;

    return texcol * _Color;
}
ENDCG

        }
    } 

    FallBack "VertexLit"
}