Comment # 2 on bug 966057 from Fabian Vogt

(In reply to Michael Andres from comment #1)
> Unfortunately rpm has nothing like a %postuntrans script, so there's no
> 'final' trigger for deleted packages after the %postun.
The issue with a %postuntrans is that the script can't depend on anything,
literally. It could be that someone ran "zypper rm glibc" or similiar, so
practically any script would fail. Even the script interpreter might be gone...

> What you can try is to execute the action immediately, if '%postun 0' (on
> final delete) was called (and no trigger has been placed by now). If the 1st
> arg is not 0 (upgrade), the newly installed packages %posttrans will do the
> job.
> 
> Not perfect, but maybe better than nothing.

Definitely better than nothing and also what I did for plymouth in my home
repo.
Issue is that this can cause mkinitrd to run multiple times during a
transaction and it also looks like it's stuck somewhere as it stays at a
certain percentage for quite a while, depending on how many kernels are in
/boot.