Yeah but you can just continue to use HTTP/1.1, which is simpler and works in more scenarios anyway (e.g. doesn't require TLS for browsers to accept it).
I have not. It is quite the niche problem. Mostly because web performance is so bad across the board that saving a few milliseconds just isn't meaningful when your page load takes more than a second and mostly is stuck in javascript anyway. Plus everybody just uses cloudflare and having that CDN layer use whatever modern tech is best is very much good enough.