Implementing (or removing) Drop on a type is a breaking change for that type's users, not the language as a whole. And only if you actually write a trait that depends on types directly implementing Drop[0].

Linux breaks internal compatibility far more often than people add or remove Drop implementations from types. There is no stability guarantee for anything other than user-mode ABI.

[0] AFAIK there is code that actually does this, but it's stuff like gc_arena using this in its derive macro to forbid you from putting Drop directly on garbage-collectable types.