If the package manager provides a function like preserve_old_lib, then it can do things like remind the user that consumers need to be rebuilt. Also, it allows the package manager to handle interactions with things like FEATURES.
But preserving doesn't work. The assumption that a library operates "on its own" simply isn't true except in special cases. The solution is better slotting, not hacks that only sometimes sort of work.
I think this one didn't ever see much love, and rebuilding consumers is handled via subslots nowadays. Closing as WONTFIX.