en.wikipedia.org
This has been a thing for years now. People were wearing masks to reduce their oxygen intake before the pandemic. It's supposed to improve your athletic performance when you're able to breathe freely again.
I hate it personally. I was stuck without a gym for almost all of 2020, so I'll put up with it for now. It's an inconvenience compared to not being able to lift at all. I'll be the first one to lift without a mask once I have the vaccine and they lift the mask mandate. I don't think most people will continue to wear masks here once the pandemic situation is better.