The first one i can think of is urgot after his rework. He was really clunky when they released the rework, then they fixed a bunch of his issues then it took about a year before he was noticed professionally (after which he was pick ban for several splits). Another example is support morgana which started being played around s4 (she was considered purely a midlaner before then). It was basically never played before then and suddenly it was pick ban despite no changes. or blue build ezreal which was invented by some gold player. or support annie which was popularized during worlds s3 by a chinese team, and heavily played/nerfed shortly thereafter. Midlane varus was popularised by POE despire no changes, there are probably tons of other examples that i can't think of because i don't have an encyclopedic knowledge of pro history. there being 140 champions in the game all with different potential roles and item builds kinda makes very likely for some strong things to go unnoticed.

I do agree that thresh almost certainly isn't sleeper op though. There's almost always going to be something strong which isn't well known, but 90% of the time it's going to be something very new and unconventional, a champion in a new role (support annie/morg for example) or a very new item build that isn't being played much. The difference between these and thresh is that thresh support is a very known thing and he's also the 4th most picked champion atm. There's nothing about tresh that's an unknown, he's popular and if he was strong he'd be played