The Forum Archetecture is quite old. Something which has nearly a decade of use is not easy to renovate and opening up a developer project to crack open established systems for the purposely of adding a link isn't a priority.
I realize it's funny in the meme-sense, but a button at the bottom of a page isn't the sherlock holmes clue that unravels the mystery.
We've advertised Discord for years via our Game, News Portal, Contests, and even supported joining it with Drops/Free Content. There's a reason why it has some 170k people on it and continues to grow.