Reportedly, players have been able to climb up on top of the wall in enclosed areas and travel through ceilings and outside of map boundaries in order to gain unintended advantages in a match. As of this writing, you cannot select Mei from the Character screen until these issues have been addressed.
Fans have since voiced their displeasure at her removal, with one person tweeting, “It’s really impressive how Blizzard has managed to unfix characters from Overwatch 1 to 2.“
Source: https://www.distractify.com/p/why-was-mei-removed-overwatch-2