It would be nice to be able to display custom warnings for posts under certain categories while typing it on the composer. Simply to remind basic rules under certain categories
something like What are Topic Templates? except for posts?
Not necessarily, it does not have to enforce anything about the structure. Just a reminer about the rules for a category. on the preview part of the post composer
/admin/customize/site_texts?q=reply_placeholder except customized per category?
That would work, but it would be much better to have it on the preview window of the compser instead of the placeholder inside the textbox
A similar topic was started recently here: Can I customize the welcome message shown when creating a new topic based on the chosen tag or category?
I don’t think there’s a built-in way to do exactly what you want today.
Topic templates (as suggested above) are probably the closest fit with developing some kind of #theme-component or something.
One option would be to include some thing in HTML comments in the topic template so it wouldn’t show up in the post even if the user doesn’t remove it, e.g.:
<!-- some tips for new topics in #category - foo - bar - baz -->