I've decided to finally get off my ass and make a proper OpenAPI spec for the api here like I've wanted to for months
You can find the source here: https://github.com/DonovanDMC/E621OpenAPI
And the rendered/usable version here: https://e621.wiki
This is intended to be helped by community contributions if people feel so inclined, so feel free to open pull requests in the repository, or mention issues/incorrect documentation you find in this topic
The documentation file is currently almost 11,000 lines so I'm sure it's got a few issues here and there