I’ve never seen someone host a wiki on Discord… that’s just stupid.
Having to join a server before you see its content is a good thing though. It’s a privacy feature and also anti-spam / anti-bots (Before you see anything you often have to agree to the server rules).
Using Discord for information storage is obviously a bad idea. But for text chat including channels, voice chat and so on it’s fantastic. Most games usually have an extra website with a wiki for information.