Let me clarify my terminology, an exploit is any form of behavior that takes advantage of the game software in a way that is not intended to gain an unfair advantage. Mei is not intended to fly (or lay on her side). Again, I find this bug hilarious, but I have already seen on social media complaints that players are abusing it.
Because making hero balance changes is a methodical process for the development team. There are older developer responses that detail this:
- Jeff Kaplan explains the process in hero balancing - Jeff Kaplan - 7/7/2017 - Old Official Forums
- Jeff Kaplan explains why sometimes heroes are unchanged for months at a time - Jeff Kaplan - 6/26/2017 - Old Official Forums
- Jeff Kaplan details the “perception of the meta” and how it affects hero balancing - Jeff Kaplan - 3/2/2017 - Old Official Forums
- Jeff Kaplan explains how they objectively balance heroes in Overwatch - Jeff Kaplan - 2/16/2017 - Old Official Forums
- Jeff Kaplan explains the Overwatch balancing triangle (Player Feedback - Statistics - Developer Opinions) - Jeff Kaplan - 1/24/2017 - Old Official Forums
Critical bugs like this need to be fixed to maintain the competitive integrity of the game, while making deliberate changes to the balance of the game can be decided over time.