Depends.
10 is pointless, it’s the same scaling wise as 11 but -1 talent, potential abilities, and free stamina.
11 is the sweet spot for a lot of good twink specs (Fury War, Resto Druid, Resto Shaman, Mistweaver Monk, Dev Evoker, DH if they skip the starting zone, Guardian Druid if they’re for some reason not playing Resto right now).
12 is worse in most cases (the talent or abilities gained have to outweigh mobs scaling to have more health and in most cases they don’t. Off the top of my head, Holy Priest often goes 12 for Holy Fire and Rapture, and I think that’s it.
13 is, again, worse, but for specs that don’t have decent abilities unlocked yet, it ends up being the best for them. So like Blood DK for Blood Boil and stuff (not sure on the specifics, don’t like DKs personally), or Surv Hunter for the Wildfire Bomb since they otherwise lack AoE in this bracket.
After that it tends to fall off pretty hard. You have a couple tank specs that like the mid-10s (generally 15-16) but with how strong the low-10s are there’s not much reason to play those specs outside of just wanting to have every potentially viable twink spec for the heck of it. This’d be like Prot Paladin, Prot Warrior, Demon Hunters in general if they don’t skip the starting zone.
By the time any character is up to 20 or so they’re just in a completely different power bracket. 20 twinking exists, but its mostly a time killer for F2P players / Alt Accounts to mess around with. 20s were fun to mess with broken trinkets for a bit, but they fixed the scaling on the more fun of the 2 broken trinkets twinks mess with, so it doesn’t have that going for it anymore (it didn’t work with 10-13s).
TL;DR no it doesn’t cut off at 12. It’s a balancing act of being as low as possible while having the strongest / most useful abilities unlocked. There’s not a huge window where leveling up is worth it, but it’s not an abrupt stops-at-12 thing.