The owner of a server sets the rules for that server. Some servers may have joint ownership through a co-operative, and some servers may consult their members for what the rules should be, but usually there is a single owner who simply decides what is appropriate.
If you go to a server’s website and click the “Learn more” link, you will be taken to that server’s info page which includes the server’s rules. It’s worth reading these before joining a server. They’re usually relatively short and written in clear plain language, so it’s not a big task.
Most servers will have rules against bigotry, abuse, threats etc, but it is totally up to a server’s owner to decide. That’s why it’s important to check a server’s rules before you sign up or transfer an account. They are not all the same and it’s best to know the kind of place you are joining.