How to exclude derivation in a package closure from the binary cache?

Development
4

Maybe derivations marked with allowSubstitutes=false should be automatically excluded from cache.nixos.org.

Obviously they still have to be built by Hydra.

But since they are marked with allowSubstitutes=false, they will never be fetched from the binary cache (unless users go out of their way to override this with always-allow-substitutes, which AFAICT was only added very recently).