My reading of the Linux signalfd man page may be optimistic. :)

Regardless, it'd be nice to have it available in the stdlib so it could be used if deemed useful.  I expect this to only ever be added by someone making use of it in another stdlib module.

As for what multiprocessing.forkserver does, the old manual signal handler and pipe trick is a reasonably well known one.  But a forkserver is not safe to be started when threads exist.  (unlike subprocess)

Signals are process global state, no thread compatible library can rightfully take ownership of a one.