As far as I know, this had a repository on GitHub, but I can't find it. What happened?

You might be looking for minibord: https://github.com/NinCollin/minibord

However, this forum is now running on a custom-built system that I developed to better suit our needs.

The timeline goes like this:

June 7th - minibord deployed
June 9th - all-nighters pulled (by me) to update the system accordingly to my needs. Editing posts, profile and post CSS customization, all of that
June 9th to July 21th - Forum is updated heavily with lots of random shit, the layout is overhauled numerous times, 3 horrific attempts at implementing a proper mod panel. Articles, more customization, what not.

Somewhere around this point, I had a feeling that we were designing something very overcomplicated. We were designing as if we were going to immediately get thousands of users overnight. My expectations were too high, and I implemented too much for me to maintain alone.

From July 21st to August 5th, I worked on a complete replacement to our minibord fork, and subsequently designed it around what I actually want the Cyberix Network to be and represent.

I've been enjoying a much more maintainable system that's specifically tailored towards my needs for a website such as this since.

The primary focus should be on the site's articles. The forum remains, but we allow anonymous posting. I care more about the messages people are saying but not the person behind it.

To be fair, we could've been able to achieve something like this in our minibord fork, but the nuclear option seemed like a better idea at the time - especially since that was my first real dip into using PHP for a site like this.