While I haven't seen it noted in any of the recent changelogs for the Steam Client Beta, it seems it has an updated Steam Runtime in need of some testing.

Sending word to us on Twitter, developer Timothee Besset said:

The beta branch of the steam client for GNU/Linux has received several updates to it's runtime for games. Make sure to test your favorite titles! CC @gamingonlinux @Plagman2

For those of you who want to ensure everyone has a good experience and to ensure games you love don't break with a new stable Steam Client release, it's time to get testing and reporting. It's probably a good idea to let Valve know of any issues on their Steam for Linux GitHub.

What is the Steam Runtime? From the GitHub:

A binary compatible runtime environment for Steam applications on Linux.

Essentially, it includes a set of libraries for game developers to link their games against. This is to give them a somewhat standard set of libs that should work across distributions for games shipped on Steam.