Comment 12 for bug 1532553

Before the introduction of systemd in Debian and Ubuntu, /etc/halt.local was not supported at all.
Neither sysvinit nor upstart executed that file/script. So I'm mildly suprised, when you say "it always worked".

We decided to not introduce support for this legacy, sysv specific service, which after a bit of research seems to be Redhat (/sbin/halt.local) and SuSE (/etc/init.d/halt.local) specific.

Adding /etc/halt.local to the mix would only confuse things.
It's much better to create a native service file with systemd, and it's much easier.