API changes that change semantics are forbidden in Standard interfaces, although APIs can be extended, backward-compatibly. To make an actual change, we need to introduce a new name. Thus, when we get around to modernizing std::vector, the fixed version will have a different name, maybe std::vec, but conceivably std2::vector.

I don't know of any plans for feature test macro analogs that would integrate with modules.

To be usable, epochs would have to apply locally to a file, not to anything #included into that file, nor to any file #including it. And, it should be possible to tie them to modules, to say "you can use this module only from code in epoch C++XX or later".