@screwtape @pkw It'd be nice for Common Lisp to have something like Emacs Lisp's make-obsolete that is actually considered by the rest of the system.

Compatibility layer is only good (mostly) if the signature doesn't change, otherwise it's creating an avoidable maintenance nightmare (especially if multiple versions are kept) while also worsening the usability with all sorts of docstring-only info (or worse, a bunch of undocumented behavior).

The pattern I've observed that is most sensible for this is encoding the difference in the classes, such that one can then simply recursively upgrade the instances (now-obsolete and unused methods can possibly be safely removed at this point depending on program flow), handling all that is necessary for compatibility differences there, and otherwise preserve everything unchanged from an external user point of view.