Riot doesn't straight up revert nerfs or buffs. That would be admitting a mistake.
Off the top of my head:
Nerf revert
- Yasuo movement speed
- Alistar W range (was nerfed due to jungle Alistar roflstomping solo queue)
- Kayle R cooldown
- Graves Q/R damage
- Ezreal passive duration, Q AD ratio
Buff revert
- Vayne E range (the buff was a "bug fix")
- Stuff about rework Eve (she actually got buffed after her rework)
- Nasus passive, R range booster
- Ziggs E minion subsequent damage (Ziggs could jungle with E)
- Tristana ASPD/lv
- Corki E mana cost
And the whole patch spaghetti that is Rengar.
There are also champs that were untouched, got a bit buffed then suddenly everyone realizes how powerful they are so the champ got a massive train of nerfs.
- Lucian: No one played him until the QoL change to Q. Suddenly pick or ban.
- Ziggs: No one played him until Riot decided to change his Q targetting, increase E minion damage and increase the center radius of R. MASSIVE nerfs afterward. He's weaker before he got buffed. I'm still salty about this.
- Teemo: frontloaded damage for E. E got nerfed. R damage and ratio got cut by like half.
- Lee Sin: Dunno the details about this one, but I remember that Riot buffed his ratios after release.
- Tristana: Because her Q isn't enough, they decided to boost her ASPD/lv to 4% (the highest along with Ashe, Cait and Vayne). Nerfbats.