sys.exit() accepts negative number and values larger than 255. I checked with strace: Python calls Linux exit_group() syscall with the value passed to sys.exit().

But then os.waitid() (waitid, not waitpid!) returns the lower 8-bits of the exit code.

In fact, the exit_group() syscall truncates the exit status:
https://github.com/torvalds/linux/blob/1a323ea5356edbb3073dc59d51b9e6b86908857d/kernel/exit.c#L895-L905

So on Linux, an exit code is always in the range [0; 255]. For example, exit_group(-1) syscall gives an exit code of 255.