If you have 1k of hard drive space, this is the perfect forum:
https://nerdparadise.com/programming/phpforum
If you can push the boat out to 5k, you have many more options, including:
https://github.com/nicbou/tinybbs
And for real extravagance you can have this at just under 10k:
https://github.com/Xeoncross/forumfive
Those aside - and the serious point is that forums aren't necessarily complicated, depending on what you want - I always disliked forums like phpbb and vbulletin, which all look the same to me, ugly and cluttered with info nobody cares about.
Since I prefer lightweight/low-end/no-nonsense stuff I always liked FluxBB:
https://fluxbb.org
a form/fork of which used to be used by Facebook in its early days (but don't let that put you off).
I really loved this one though:
NoNonsense Forum
Both of those are ancient but still around/seemingly unhacked/exploited and pretty lightweight.
If I'd ever got around to doing a forum they're the two I'd have chosen from, or maybe just written a bloated 15k one myself.