You cannot post illegal material, doxx, harass users, or post malicious links.
If you can't do it in the United States or Germany, you can't do it here.
Like... please? If you're going to shit post at least put some effort into it. If there is already a thread where your post belongs, use it!
If you really need to post /trash/, there is a board just for you.
The problem is never the plug; the low effort is the problem. If you're going to post a quality post in the correct thread that includes links to your stuff, I'm very inclined to allow it. But otherwise fuck off.
Do not post things that are intended to disgust people: gore, filth, etc. Use your common sense.
The best way to be sure is to just reach out. Ask on /meta/ if you don't know another way.