Valve continues to port their back catalogue to Linux even as its market share on Steam declines. It might be easy to declare gaming on Linux dead, or something like that, but the platform has not yet been pushed for gaming. It is entirely possible, albeit increasingly unlikely (and that is bad), that Microsoft will continue to support an open PC gaming experience. If it continues to sink then Valve might see more appreciation for their work.
Linux gamers of today, however, can access a beta build of Half Life: Source. If this seems oddly familiar then you are probably thinking of Half Life which, itself, was ported to Mac and Linux last January. The Half Life: Source beta announcement came on September 12th.
Not only has Valve kept their 15-year-old game up to date with current hardware and alternative operating systems, they are actively keeping multiple versions of that 15-year-old game up to date with current hardware and alternative operating systems. This is the classic PC gaming mentality also seen in Blizzard and, until a few years ago, Epic Games.
This beta release is not just limited to Linux and Mac, however. Valve encourages users, of all platforms, to test the product and reports bugs to their GitHub.