It depends entirely on whether there are still developers interested in maintaining support for those architectures. It seems pretty safe to assume that Torvalds isn't going to be in favour of "we drop anything without a rust backend tomorrow"; but on the other hand 8 archs were dropped in 2018 -- https://lwn.net/Articles/749292/ -- essentially for lack of any active interest in them. It's possible that eventually "set of archs with a rust backend" and "set of archs which people care about enough to keep alive in the kernel" converge to the same set. It's even possible that one day "does this even have a rust backend?" becomes a marker of "is this thing really of use to anybody" in the same way that in 2018 "does this thing have support in upstream gcc" was a marker for "maybe we can drop this". That would be years from now, if ever, though, IMHO.

Linux removes platform support from time to time, like any project. IA64 was a recent, notable example.

That being said, it is true that Rust doesn't yet support all the platforms that are important to the kernel. That's one of the reasons this is starting with drivers, and is one of the reasons why some folks are pursuing a Rust frontend for gcc.

Plenty of them have already gone away. If they don't have people maintaining them, they get dropped over time.

I thought that was NetBSD's thing?