Except in every other competitive shooter, this has never been an issue.
There are a huge amount of games in the Esports circuit right now that support 21:9 resolutions and there's not a single complaint to be heard, or any sign of players going out of their way to use it.
Even bringing up CSGO again, you have people choosing to play at 4:3 resolutions and then just straight stretching them out on a 16:9 monitor. They actually willingly sacrifice their FOV (due to the scaling they gain little to no vertical, but lose some horizontal) to play this way.
No one who matters in a competitive sense cares. The only people who actually care ARE the casual people who play competitive, the ones who perceive any and every perceived advantage someone might have for the reason they lost. An actual pro player isn't going to make a fuss about this.
This is one of those times where people are making a huge amount of assumptions to what the competitive scene actually cares about, and a huge amount of players new to the FPS genre (Who likely transitioned over from other Blizzard titles), or just people who seem to make it personal when you insult their company of choice, who don't understand how little of a deal 21:9 actually is.