Upgrade to 1.19.1 removes /usr/sbin/start_pms

That’s not right. It has never failed to detect Init on Ubuntu.
I have Ubuntu on my desktop (19.10 at present).
I have all the supported Ubuntu and Debian versions in VM.

Which actual 1.19.1 DEB did you install ?

I’ll give you the proper /et/init.d/plexmediaserver. I’m more concerned with why it went sideways at this point.