Tell me where fiction is illegal to publish

In legal terms it is (with the obvious exceptions for impersonation, defamation, etc)

But in private forums? Mods can be however strict they want

Absolutely not you dimwit.

I run a security related subreddit (cryptography). A false sense of security from wrong answers can literally kill

Way ahead of you, our reddit forum mentions the difference in every single place we can put a custom text, definitely intending to do the same here