The POSIX threading model does include the ability to send a signal to a particular thread using pthread_kill().
This is not what is being suggested. Read about the semantics of thread killing in C# and Java.
While Yonatan is talking about the old pthread_kill function rather than the modern one you suggested, there are still similar issues. It still works by sending a signal under the covers, it just provides a much nicer API (but one that nobody uses—and in particular, Python doesn’t) to handle that signal indirectly, by managing thread cancellation state and type and cleanup functions.