> That is literally impossible; a breaking change from X-1 to X+1 must have been introduced in one of the intervals X-1 to X or X to X+1. It the latter, X+1 breaks compatibility, if the former, let X' = X-1 and X'+1 breaks compatibility.

No, that's not how it works. The deprecation is introduced in version X-1; the new behavior is introduced but is opt-in; in version X, the new behavior becomes opt-out; in version X+1 the old behavior is removed.