With a few exceptions (mainly where hooking into the kernel is concerned), deprecated iOS/macOS APIs tend to hang around for many years after deprecation.
If I coax a random FOSS Mac app project from the early to mid 2000s into compiling on modern macOS there’s usually deprecation warnings left and right, with many of the deprecations having happened in OS X 10.2-10.6 (well over a decade ago), but it’ll still work fine.
So devs usually have plenty of chance to move things over. The grace period just isn’t indefinite like it is with e.g. Windows — sometimes the deprecated APIs are removed 5-6 releases down the road and so it’s a good idea to move off of them sooner than later.
If I coax a random FOSS Mac app project from the early to mid 2000s into compiling on modern macOS there’s usually deprecation warnings left and right, with many of the deprecations having happened in OS X 10.2-10.6 (well over a decade ago), but it’ll still work fine.
So devs usually have plenty of chance to move things over. The grace period just isn’t indefinite like it is with e.g. Windows — sometimes the deprecated APIs are removed 5-6 releases down the road and so it’s a good idea to move off of them sooner than later.