Probably not the case the other way around.

Actually the other way around, new pip version uses new package dependency resolver that can concluded that a previous package setup is not supported (because of version conflicts) even though they worked...
It is tricky, pip is trying to get better at resolving package dependencies, but it means that old resolutions might not work which would mean old environments cannot be resorted (or "broken" env). This is the main reason not to move to pip v21+ ...