I've been happy with Request Tracker for over two decades, but the scope of RT is also a lot smaller than ZenDesk. Just email-first ticketing, little more.
However RT is really just a request tracker and doesn’t do some of the fancier pseudo PM stuff zen desk does. That’s a plus in my book since when I’m tracking tickets I don’t want a JIRA clone but worse.
Also it’s worth noting I haven’t used it in 10 years so it might have changed substantially since then.