Every part of API may change at some point - that's unavoidable. The question is - when. If API is public it is rather unlikely that it'll change
(we're talking about backward incompatible changes) during entire 4.x, 5.x, etc branch life. However, some changes may be required to fix a bug or introduce new a feature. We're avoiding them, but sometimes there's no other reasonable option, so the change is made in major release (4.3, 4.4, etc.). Note that we always mention them in the change log. Then, in minor releases (4.3.1, 4.3.2, etc.) backward incompatible API changes are really rare, so currently I cannot even think of any. But that's about public APIs.

The APIs marked as private may be changed regardless of these rules.

However, in undo manager case the API is split into two parts - public events (editor#saveSnapshot/lockSnapshot/unlockSnapshot/updateSnapshot) and private methods. So there's actually no need to use the class directly. And since events are public, their behaviour will not be changed without any serious reason. Actually, in this case, changes are nearly impossible, because those events are used by hundreds of plugins.