Have you ever screamed in frustration at the monitor cause that champion's kit is just too OP? Well, the good news is, there are some that could've been even worse, but aren't.
At some point, Olaf had a special passive called "Ragnarok". When he hit 18, or maybe some other criteria were met, a percent max HP true damage DOT was applied to all champions on the map, slowly draining away their health.
Jinx ult at one point had an AoE polymorph attached to it. Imagine having to face that with her recent Arcane buffs.
Moving on to assassins, can you imagine that the previous version of Ekko's ult rewound the entire game by ~4 seconds, excluding champions' deaths. Cooldowns, mana/health, position, everything for everyone. There was a tamer version that applied only to him and 1 targeted champion, but that didn't make it to live either.
Fiora's Ult was supposed to work similarly to Mordekaiser's R where she takes her target to a place for dueling.
There are even rumors floating around that Aphelios' was supposed to have up to 25 rifles in his arsenal. Of course, these are just rumors and jokes between developers, but it just goes to show what type of thinking Riot developers have.
We certainly wouldn't want to face any of these champions on the Rift, and we are sure that there are more such examples of overpowered abilities never making it to the live servers, or PBE for that matter.