Discord is (or at least was) easy to "implement" because their bot and user API is mostly the same.

until they ban you under the ToS that says “no third party clients”.

https://news.ycombinator.com/item?id=28435490

A bot or a bridge isn't a "client".

They just don't want to fight people trying to build a full alternative client for Discord as a bunch of their paid-for stuff is just clien side javascript.