Exactly like Lee says, you can have pretty much any scene component draw into a decal stencil that opts them out of decal rendering. (including the niagara component itself)
Glad Niagara does actual decals now so you can omit them from projecting on things. I never wanted to use the faked decal method in UE4 because you couldn’t omit things without a handful of workarounds
I don’t know if they have been added recently in UE5, but the latest NS properties have so many useful options like the custom depth, but it would be even better if we can do this on a per-emitter level.