It's still quite commonplace to end up with (libraries (... bytes ...)) even though using Dune implies OCaml >= 4.02.3.

It would be better if Dune dropped the dependency and issued a warning (especially now that we don't depend on findlib, requiring a dependency on base-bytes in opam-repsository).

EDIT: It would be better if we had such a warning for all libraries in fact. This should be possible via ocamlobjinfo.