Why there is no GNU+linux os mentioned in github of julia instead all windows?

As Opensource community, I feel sad when i don’t see mentioning gnu/linux os in github readme of julia.
Instead using some Proprietary os in priority.

I think you’re misreading this part of the readme - that sentence is mainly there because the default Windows terminals (cmd/PowerShell) are severly lacking in features such as Unicode support and thereby make it harder to get the most out of the Julia REPL. On Linux this isn’t usually an issue, as the terminals provide much richer functionality, so no specific recommendation is required.