Is this a rule?

I don’t know if it’s allowed to advertise a server.

No, it is not allowed, unless it is in a private message and not sent to a load of people (like 2 maximum) and they have permission to by a staff member. However you can hint to a server, but not say the IP or something close (for example UnraveledMC Com or UnraveledMC(dot)com)

You could probably mention a hint to a server name and not get in trouble, however if you are to say a servers name or IP that’s a issue… and or both.