I'm not affiliated with the people running this so I can only comment as another user.

There's nothing in the official rules about adult content but adult content also is not one of the ways which improve the chance of your game to win. The questions you need to answer with your game are: is it fun? does it integrate web monetization interledger tech well? does the game fit the "secret" theme well?

You should use common sense in terms of content and keep it generally safe for work since the reviewers are all professional people who are volunteering their time to judge the games. I think not including anything Apple or Google would block your game for if you published on their stores would be a safe bar to stay above.

Since you must post on itch you need to follow their rules which allow anything that is not illegal. However if you want to monetize it you have to follow the rules of the payment providers.

https://itch.io/docs/creators/faq#is-adult-content-allowed

Coil also has a list of restricted businesses,  as they use Stripe for payments which has that same list. One on the list does prohibit certain types of adult content, but it wouldn't prohibit selling a game like Grand Theft Auto since even if it often has a strip bar and other adult content inside of it it's only a small element of the whole game.

Adult Content and Services

Pornography and other obscene materials (including literature, imagery and other media) depicting nudity or explicitly sexual acts; sites offering any sexually-related services such as prostitution, escorts, adult pay-per view, adult live chat features; sexually oriented items (e.g., adult toys); adult video stores and sexually oriented massage parlors; gentleman’s clubs, topless bars, and strip clubs; and sexually oriented dating services.

https://help.coil.com/general-info/terms-of-service

https://stripe.com/restricted-businesses

Something else there is also important "encouraging users to idle to reach a specific amount paid" as people may be tempted to make a game which rewards idle time playing (you could still reward something a daily unlock).

The Coil rules are hard to enforce from a payment pointer point of view but since they are involved in the process for most people participating in this gamejam you should be considerate of them and not make anything they would frown on.