Tomorrow, to celebrate Zelda's anniversary, Ganon, the King of Evil will join Super Smash Bros Ultimate, breaking the "bosses can't be fighters" fan-rule.
Fan-rule that anyway was already broken since Melee, where Bowser was both a boss (Giga Bowser) and a playable character, but still.
Eh, that's kind of an awkward one. Giga Bowser is a revision of Giant Bowser but actually is a Smash OC officially. It was broken in Brawl as a Final Smash at best, though.
So it didn't really break the fan-rule.
Also, it'd have to be a downsized OOT Ganon to really break it, since he's also a Final Smash. That's an actual non-Smash OC in this case. Besides that, it was broken when you could play as Master Hand(even for a short while), a more proper one.
That's kind of the issue with fanrules anyway. Exactly what really counts as breaking it. Plus, the supporting is for the version of Ganon to not appear in Smash in any way, ALTTP, who is vastly different from OOT Ganon is both design and abilities.
It's also why fanrules just plain suck to deal with. It's hard to consider it broken for a lot of people, so.