A simple stencil buffer masking shader for Unity

UnityStencilMask.shader

Shader "Custom/StencilMask" {
	Properties {
		_StencilMask("Stencil mask", Int) = 0
	}

	SubShader {
		Tags {
			"RenderType" = "Opaque"
			"Queue" = "Geometry-100"
		}

		ColorMask 0
		ZWrite off

		Stencil {
			Ref[_StencilMask]
			Comp always
			Pass replace
		}

		Pass {
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag

			struct appdata {
				float4 vertex : POSITION;
			};

			struct v2f {
				float4 pos : SV_POSITION;
			};

			v2f vert(appdata v) {
				v2f o;
				o.pos = UnityObjectToClipPos(v.vertex);
				return o;
			}

			half4 frag(v2f i) : COLOR {
				return half4(1, 1, 0, 1);
			}

			ENDCG
		}
	}
}

If you are using URP and the masking effect isn't happening, or your stencil shaders are giving a parse error claiming there is an unexptect "}", you can fix it by creating a new URP shader graph (if this is for the stencil object shader, set up sprite color & alpha outputs from _MainTex inside the shader graph editor, so that the resulting material can get the color of your main texture), click on "View Generated Shader" in the inspector, copy the generated codes from the shader graph to your .shade file, then make the subshader edits here. As long as both of your shaders are URP, things will work properly.
@RandomVariable1470