Bug tracking for the Steam for Linux client has been moved to GitHub. Steam for Linux now has a public repository and an issue tracking system for creating or browsing bugs related to the Steam for Linux client. You will need an existing GitHub account[github.com] to create a new bug[github.com]. For more information on creating a new bug, see https://github.com/ValveSoftware/steam-for-linux/blob/master/README.md, located in the public repository.
Note that this is a bug reporting area for Steam itself, not games. Game bugs should be reported to the game developers.