> But it's worth noting pthread_cancel() is also rarely supported anywhere outside first-class pthreads-capable systems like modern linux

Having written some of the implementation for a non x86 commercial Unix well over 30 years ago now (yeah, I know), pthread_cancel is not that rare. A carve out like “modern linux” is io_uring or even inotify and epoll. AIX and HP-UX, fuck even OSF/1 had pthread_cancel.

Windows has TerminateThread. Most RTOS have some kind of thread level task killing interface.

While they have different semantics than pthread_cancel, that doesn’t really affect the example you’re giving - they can all be used for the “cpu-bound worker”