Debian, Ubuntu, and Mint are over half, and Arch & gang are about a 1/4
1. Officially, Proton is only supported on Ubuntu. It doesn't stop you from trying though and many Arch-based OS reports are just as successful.
2. Reports people submit includes pasting Steam system info, which for some Arch installations reports as 'Linux 4.x' unless they install additional an additional library.
If you're feeling generous, count the 'Unknown' category as Arch-based.