A little surprised that the linked systemd file-hierarchy(7) manpage makes no mention of /opt
* 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§ion=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.
But sure, absolutely, an installer should prompt the user for an install location, and I think a default under /opt is probably among the best defaults possible, if we consider installing outside $HOME to be reasonable.