I've used Request Tracker for years. It's not pretty, it's written in Perl, but I can fairly easily make it do all the ticket tracking flows I care about and it just runs and runs and runs. My scale is admittedly small, but I put tens of thousands of tickets per year through my instance, and i basically never have to touch it unless I'm setting up a new queue or different flow for something.
Wow, I’ve never seen anyone mention RT here. I used it for years when I was working IT for my university while in undergrad. It worked pretty well. It didn’t have a lot of features but it allowed clients/customers to respond to tickets via email which was pretty cool at the time (late 00s). It also ran pretty fast on the terrible servers we had it on.
We still run it today; they had a major release last year, I think. Its key feature is that it remains email-first. Customers never interface with the website, for them it's all just like they're emailing a human, with some extra tooling and tracking on top.
