Re: Functions in kill-emacs-hook aren't run if emacs gets killed with SIGTERM

[Richard M Stallman]

The main difference between SIGUSRx and SIGTERM is that SIGTERM is the
normal way to stop a process, while SIGUSRx are meant for
communication to it.

So it seems to me that Emacs should pass SIGUSRx to Lisp code and
otherwise ignore it.  I believe that's what it did in the past.  At
some point it pushed an event which could run a command later.