It's nothing new for a recently released champion to not actually see the light of the Rift until heavy nerfs have been given. Bel'Veth is in her own category though.
Every new champion in League of Legends, one they are released is either a pick or ban for the foreseeable future, until a more broken one appears, or Riot nerf them into the ground when skin sales drop.
Bel'Veth is no different. One of the most hyped champions in the game, she was released in patch 12.11, and, unlike other champions that got banned so no one gets to play them, Bel'Veth is broken enough that even beginners can carry games on her.
She, currently, has a 53.19% win rate across all ranks, while her ban rate is 64.5% across all ranks. in addition, her dashes are enough to outrun movement Speed buffed Miss Fortune while taking no discernible damage from ADCs, due to her healing.
There are a number of reasons for that, but the main one is, of course, her E ability, which offers her 70% damage reduction at level 1. For comparison, Irelia W reduces 40% physical damage at level 1 and doesn't also heal the champion, while doing damage during the duration of the ability, nor does it give conqueror, or lethal tempo stacks.
Add to that the fact that she scales incredibly well, and you have a better Master Yi, with a Sylas W levels of a broken basic ability, which makes this champion OP, even with her disjointed design.