So… Epic being epic, I’ve tried in 5.4.3 and if you left empty any of the material inputs then they didn’t get written.
BUT! if you are defining any property on a set material attribute node, or INSIDE A FUNCTION, those would get written. This said, If you create an empty material, and just plug in a normal texture as output, it should work as a normal decal only.
For some reason the compiler just recursively explores all functions but doesn’t checks if they are finally connected or no.
Another workaround if epic would like to fix this, would be to give the community control over each independent GBuffer alpha channel.
They already store three float4 buffers, BC, Normal, and packed Metal/Specular/Roughness. and they just get the opacity input and use it for every alpha channel of those buffers instead of give the option to define them separately, which, would also fix the issue by giving control to the users.
So it’s quite an easy fix, just need to see if they fix it