A little surprised that the linked systemd file-hierarchy(7) manpage makes no mention of /opt

You won't find it in the hier(7) manual pages on BSDs, either.

* https://man.openbsd.org/hier

* https://man.netbsd.org/hier.7

* https://man.freebsd.org/cgi/man.cgi?query=hier&sektion=7

* https://man.dragonflybsd.org/?command=hier&section=7

There was a long time when the Linux world held /opt in disfavour, because officially it required either a stock market ticker name or some other corporate identity to make a subdirectory legitimate. You can still see traces of this in the Solaris descendant operating systems, where pkginfo(5) talks about package names using corporate stock ticker names.

* https://illumos.org/man/5/pkginfo

/opt/SUNW* used to be a very familiar thing to a lot of people.

Maybe enough time has passed for anti-corporate memory to fade. Maybe there's enough corporate backing in the Linux world now to resurrect the idea regardless.

Maybe /opt/RHT* is the shape of things to come. (-:

I've never over the years seen the systemd people advocate for /opt, though.

/opt/ is just a dumping ground for crap nobody can find a better location for

/opt is used for manually-installed software. Packages should never place files there, so it falls out-of-scope for file-hierarchy(7) or hier(7).