I've been thinking on-and-off about creating something like this, and today discovered someone else already did! Which is fantastic.

frozensnowfalls, a community that archives old LJ prompt communities. It looks like they archived 219 different challenges! The easiest way to browse is from the tags page.

I might have spotted a couple challenges I started writing for over 15 years ago and never completed...

It makes me so incredibly sad to see how many people in fandom see generative AI in a positive light and openly make use of it to "write" fanfiction.