Crosby, Stills, Nash and sometimes Young. Cream. Those are some earlier examples of "Supergroups".
Had to check dates, but Cream is a good example, had completely forgotten about them. Also fantastic stuff there. I don't know why, but thought the Travelling Wilburys were much earlier than they ended up being (late 80s, I'd have put them in the 70s easily). Clapton was previously big with other bands, I'm struggling with the other two. Both were good/known, but were their previous efforts 'big' enough to qualify? Sure they were big in their circles, but I really only know the other two BECAUSE of Cream. maybe they were bigger in blues circles that I don't follow in that era. to be fair, I wasn't born until 15 years after Cream got together.

Crosby,Stills,Nash & Young probably technically qualifies, just not sure it gets to 'super group' in my head. At least not at the time they got together. Yes, they were parts of other bands that had success (Byrds, Hollies, Buffalo Springfield), but I see their biggest contribution as being part of CSNY, not the earlier efforts. Maybe it's because their combined effort was so good? Afterwards, they were bigger names in my mind, and their solo projects went well, played together again, etc. Supergroup NOW, but were they big enough at the time?
Travelling Wilburys just have that overpowering SuperGroup feel. I mean, the least famous guy in the group was probably Jeff Lynne, and he was the frontman for ELO, and he seems like he organized/ran the whole thing. Harrison, Roy Orbison, Tom Petty, Bob Dylan; that's an insane amount of musical talent in one place. Even looking at a big wiki list, not seeing another band that is at their level. A couple kinda close with newer bands, one or two that was basically just an existing band with a new singer.
Plastic Ono band is actually a decent example: all the Beatles except Paul, Eric Clapton played with them, guys from Manfred Mann, the Who, and Yes. Solid, Still not really scratching Travelling Wilburys, they've got no one you have to look up, every member is RnR Hall of Fame on their own merits. It's really pretty amazing.