Not sure it’s so simple. I think close to 100% of new ambitious projects are going to leverage AI at least to some degree. I know a couple that have strict no-AI policies (e.g. Zig), but it’s a tiny minority i think.

So how much AI usage does it make it an “AI rewrite”?

Dunno. I got rather the impression that it's ambitious single-developer projects with no intention of maintenance which leverage those 'AI' code generators the most.

Who wants to contribute to an unmaintainable code base?

When the majority of the code is written by AI, it is more than 50%.