For an on prem alternative to Jira, you can have a look at Tuleap.
We (I'm from the dev team) don't aims to re-do a "simple" alternative to jira because there are tons of them and they fall short when things to track get more complex. You can do simple (as github) you can do hellish (as jira) but unlike jira, you don't mandate one model over another (no hundreds of un-necessary fields because someone asked them once).
It's GPL, there is an entreprise plan, it's available on prem and cloud.
I'm part of the dev team so take this comment with the level of distance you want but you should have a look at Tuleap [1].
You get the customization to yet another level as you are not constraint by the fact that templates are shared across your organization. You can tweak and customize every single tracker (issue type) with it's own fields & values without impacting others and without depending on an admin for that. You can deep dive with https://blog.tuleap.org/tuleap-versus-jira-software/
If you likes gitlab and look for better planning capabilities you can have a look at Tuleap [1].
Tuleap shines with very advanced tracking capabilities and, most important, empower end users to manage it. Unlike Jira, you don't depend on a central admin to tweak you configuration, everything is at hand.
Git & CI capabilities are built-in but if you prefer gitlab for that, you will have soon an integration between the 2 tools. It's part of the next delivery [2] due mid november.
If you're looking for a powerful tracking system, I'll recommend Tuleap[1] and esp. the "Trackers"[2]. If you need to build custom workflows or to adapt tool to an existing internal workflow, it's a tool of choice. We (I'm part of the dev team...) make a strong stand to make the tool adaptable to organizations instead of forcing orgs to adapt to the tool. Oh, and it's open source ;)
Most PM tools are designed to have a somehow central enforcement of the process and workflow with some central admins that have ways to change that (permissions, fields, workflows & all). This cannot be anything but a failure if the central admin doesn't say NO to 99% of modification requests because there will always be this influential project manager that will come to "add this little field I need to track this stuff for $IMPORTANT_CUSTOMER". So, by design, it will either be bloated (everyone get it's change so bug template is fat) or useless because the form will be so simple that everything will need to be managed with text content in comments.
That's why we (Tuleap team) propose an alternative:
- You want a simple -github like- issue tracker because you are 3 in your team and just a title and a description, please go ahead.
- You want a full blown, CMMI Level 5, Spice 3, what not issue, requirement, risk tracker, please let the craziest process guy do.
What's the difference with Jira (and others) ?
Each "tracker" (issue definition) is local to one project (of course you can template for re-use) and 100% owned by the project. You are not limited to 3 or 4 central templates that nobody can modify, you own them.
Let say the "simple" team is mad of tracking which component of the application an issue is raised on or how much effort was need to complete the issue, in 30 seconds the template is modified and usable without impacting anyone else.
Very good agile support (Scrum, kanban, hybrid) as well as development tools (git, pr, ci).