I remember reading in the rules channel that the server was 17+ only, being the reason why i left it. Although i'm not entirely sure if people within the age of 17 and above can enter the server, or only those above 17 years old.
Saying your game is for players 17+ only and having the game's Discord be age-gated are two entirely different things. A lot of people do not want minors in their Discords for a lot of perfectly good reasons. Moderating a server becomes way more of a responsibility when minors are involved and most people i know who have a server just don't want that put on them so they make a "no minors" rule even if posting NSFW content is also against the rules.