WoW has been doing this for years themselves, though.

Remember Vanilla with no inherent guides for questing? Now there's built in HUD features and GPS markers for that to more or less hold your hand the whole way rather than the player having to <shock, horror> read the quest text which says "Go kill some boars, there's some to the East".

Atlas Loot was extremely popular for multiple expansions and then either in MoP or WoD [I didn't play MoP] they brought in the official version. Which was so much worse and missing a lot of features like wishlists.

Various feedback tools like floating numbers for damage/healing done became built in.

LFG was also a mod in its own ways at times before and after the official version because again the official release sucked. Blizz even disabled some features so LFG mods couldn't work as they originally did like for speeding up world quests by auto adding/joining a group to blitz a world quest area in super fast time.

I know I had a specific ui add on for my resto Druid that I would have to turn off for my other classes.

Decursive is probably one of the mods I remember most for how much easier it made dealing with shit across various classes due to differing cleanse abilities. Would often cause trouble however like when Spellsteal from mages would result in the mage becoming Mind Controlled because of what you stole 🙄