Mew was so good in DPP Ubers that there was a brief(?) period where people were whining about it being banned in Ubers. Someone who was more immersed in 4th gen Ubers could probably say more, but my understanding is that it was mostly a Baton Pass thing, I thiiiiiink combined with Deoxys-S dual screen lead? In any case yeah, I think Mew being OU was just never even a question except for really early on (where people were bringing up all sorts of stuff willy-nilly like Ho-Oh and Darkrai)
I don't know dates and things for 4th gen ban process but
Cathy/Amazing Ampharos era: Wobbuffet and Garchomp allowed
eventually Deoxys-S gets tested, and I think after that is when Smogon gets fed up, gets an official server and starts thinking about developing some sort of formalized process. Wobb/Garchomp/Deoxys get banned and we create some big weird list of things we plan on testing (which includes stuff like OHKOs I think. Also species clause for some reason?). It's weird though because after testing the stuff individually, the plan was to test all of them together. I don't think that actually ended up happening though.
I wanna say the Latis were after Garchomp, then Platinum happened and Skymin became a thing, so banning that became a priority for people. After that my activity dwindled so I have even worse memory for it; I don't remember if Manaphy was tested at all (I know we planned to initially but I don't recall the actual test).
There's probably a thread in Policy Review or something that has more reliable info though!