Application extensions outside the Apple ecosystem have been used on Apple computers for decades. It was never limited to Apple provided systems, whatever that means.
Having been (a minor) part of the Firefox effort, that was a real pain to implement, so sounds like a very useful component.
It's nice when extensions don't crash the host application, but the mitigations aren't free. It's interesting that Swift's stable ABI will drastically lower that cost, since you usually need some kind of serialization (although people have been using mmap to share data between host/extension process for awhile too to avoid the serialization penalty)