(Disclaimer: I’m not an industrial user)
RankNTypes was infamously changed recently which broke a whole lot of code and spawned the new DeepSubsumption extension that undoes the change. Also, ExistentialQuantification has changed and will probably soon change again, see the other thread (that is a rather trivial change, but it could break code).
GHC2021 has become the default. Removing old extensions just breaks existing code for little reason. I guess you could argue it would improve discoverability of the remaining extensions, but that doesn’t weigh up against the breakage it would cause; the categorisation proposed here would be a better way to improve discoverability in my opinion.