Shortly after drastically improving the performance in the Linux version, Crowbar Collective actually released the Xen chapter of Black Mesa out to everyone.
You no longer need to opt into any Beta, plus it comes with updates to the gameplay gathered from feedback during the beta period to "refine the difficulty, increase player guidance, fix bugs, as well as increase performance throughout the Interloper chapter". Black Mesa is still in Early Access, but at least now you can complete it and you should have a pretty good experience but they will still be taking on feedback and adding in some more polish to it.
With the release of Xen, they've also shared a roadmap of what's coming up. With one of the next releases, they're going to be adding in a new set of Steam Achievements for the Xen levels which they have ready to go. With the big "1.0" release they're looking at doing a pass over the multiplayer to ensure it's all working, they're going to take a look at the AI and see if they can bring the improvements from Xen over to the Earth levels, ensure Steam Workshop is working correctly and overall bug fixing.
Once Black Mesa leaves Early Access, they're planning to go over the Earth levels again to ensure they're using all their fancy new graphical features (likely made for Xen), more gameplay improvements and ensuring it all just fits together properly. They're joking calling the post-EA update "Black Mesa: Definitive Edition".
You can pick up Black Mesa now on Steam where it's 20% off until January 2nd.