Is AtomicU128 gone for good?

I keep seeing the AtomicU128 type come and go in std::sync::atomic on a semi-regular basis (I think I've seen it be put in and removed at least twice, though it could have been more than that). Why is it being removed each time? What's preventing it from becoming a permanent part of the standard library?

AtomicU128 is only available on targets that have native 128bit atomic operation support. This includes x86_64 macOS (as the minimal supported CPU by the oldest x86_64 macOS (Mac OS X Tiger), the intel core duo, supports them.), but not x86_64 Windows and Linux as they work on CPU's that don't support 128bit atomic operations.

macOS:

Windows:

Linux:

Ah, so adding it in was an accident each time? Unfortunate; it would be handy for some algorithms that I'm writing (not required, just handy).