Mei was disabled in Overwatch 2 due to a bug with her Ice Wall ability. The developers removed her on November 1 and explained the decision on Blizzard CS The Americas Twitter.
This is the second time a hero has been disabled in Overwatch since the sequel's release. After the fix, Mei will return with the upcoming patch on November 15.
We are temporarily disabling Mei to address a bug with her Ice Wall ability that allowed heroes to reach unintended locations when used with their abilities. We are working to address these issues as quickly as possible and aim to bring Mei back in our next upcoming patch which is set for November 15. Thank you for your understanding and patience.
Before that, Blizzard removed Bastion and Torbjörn from the game due to multiple bugs. They blocked the heroes on October 10 and brought them back last week.
The next patch will arrive on November 15 and will nerf several characters, including Zarya, Genji, and Sombra.