The docs for Decals say that they aren't available for the Compatibility renderer, and to consider using Sprite3D instead. But I can't find any examples of how to do actually this. Does anyone know? 😃

Edit: So this works, but only if the "projector" mesh is below the camera, and it will re-texture the bottom of an object as well which is pretty bad for maps with verticality. Though I don't know if that's how regular decals work as well.
https://godotshaders.com/shader/decal-shader-for-4-3-compatibility-renderer-transparency-support-no-repeat/