Back then it was only Annie. She essentially became overpowered the same way Zed did: although she didn't receive any buffs, a lot of the mid lane champions were nerfed, making her stronger in comparison.
In mid lane, she could decimate popular picks such as Yasuo or Viktor. On top of that, she was already a well established counter to assassins such as Zed. In the bot lane, she could easily out trade enemy supports due to her high base damage, and because she could hold her stun pretty much indefinitely as support, she is basically guaranteed to win any skirmish.
However, that was before Riot buffed Veigar, and reworked GP, Darius, Garen, Mordekaiser, Skarner and Fiora.
Annie's 100-0 potential isn't that overpowered anymore simply because so many other champions can do it too. Heck, Veigar doesn't even need a full combo to 100-0 someone.
As well, the reworked champions either rivaled Annie in CC or burst. A GP's barrels were practically multiple Tibber ults. Fiora could block Annie's CC and bring the base fountain to the team fight. Skarner had both damage and CC like Annie, but he also had the benefit of being tanky.
People stopped complaining about Annie simply because there are worse things in the game. She is still a pretty solid pick and is still definitely one of the strongest supports/mid laner.