This is correct. We do it to keep things fair.

I'm curious, is this actually enforced? If so, how? It seems like it would be trivial to attend with a pre-built project if one were so inclined.

Anyone interested in helping build something to check this? That would be a pretty meta hackathon project. Probably not the most promising business idea though...

There was a project I started a few years ago but abandoned. Is it fair to do that project again if I’m starting with a new codebase?

You start from scratch. At least that's what I've been following for the numerous hackathons I've attended/won/lost.

Of course no one's going to check if you worked on it last night or whatnot (I know friends who've taken previous hackathon submissions and submit it to hackathons like it's new and actually won) but for the hackathon spirit you shouldn't.

That's the officiallyunofficial rule at all the hackathons I've been to, but also at every hackathon I've been to, the biggest prize winners were teams that were coming in with something they'd obviously been planning and working on for at least a couple weeks ramping up to the hackathon.

I imagine with a ycombinator interview on the line that sort of behavior will only be more prevalent.

I can imagine a startup which applied to YC W20 coming waiting for pre-seed XD.