I complained about the "not allowed" message half a year ago. It seems safe to say that they do not care. It's even worse in NSFW communities, where it will actually tell people to post their content in another (SFW) community instead, and many users do, in fact, do just that.

Guideline text is completely useless, almost nobody will ever see it. Many people have reported this many times; Reddit does not care. Putting that text in a pinned post or the sidebar (and hoping people will find and read it), along with making your removal messages concise enough to still make sense for people who haven't, is the best you can do. Put a link to message the mods there too, because that option is also really hard to find in the app, and I don't know if that was ever fixed either.

The post requirements are generally useless, as you won't know there's a problem, and the vague message to the user will not help them figure out that's wrong. Most users will just give up and post elsewhere. You're way better off using Automod to enforce those requirements, as you can set each requirement to give a concise removal reason, and you can see the removed posts to see what users are getting wrong or if your requirements are interacting in an unintended way. Bots can handle this even better, if you're able to write one.