A community rep explained the reason behind the Overwatch 2 patch delay:
Today’s patch is delayed as we work to resolve a critical issue. We’ll provide an update here and set the patch live as soon as the problem is addressed.
This includes the upcoming balance changes, bug fixes, including those that affect Mei, and core content updates we have planned for this patch. This delay also impacts the delivery of Overwatch League Viewership Incentives & Perks from the Postseason matches during the period between Oct 30 - Nov 4, along with the rotation of cosmetics in the Overwatch League shop.
Daily and weekly challenges will reset today, as well as the standard shop refresh.
Thanks for your understanding, we’ll be working on releasing this patch as soon as possible. Follow our forums and socials at @PlayOverwatch for updates.
Blizzard temporarily removed Mei, one of the OG Overwatch heroes, from the game a few weeks ago because players kept abusing a glitch that let her go out-of-bounds using Ice Wall. This exploit allowed players to hide from plain sight. But, while Mei is the focus of the delayed patch, Blizzard will address several issues in Overwatch 2 as well. More importantly, the patch will also add more "core content updates" that the studio already had planned.
Blizzard later confirmed that Mei will return as a playable character on Thursday, November 17, at 11:00 AM PT.
Speaking of Overwatch 2, the game might get banned in Russia soon following a petition to make games that push "LGBT propaganda" illegal.