Not that much, really - which is hardly a surprise. Developers Crowbar Collective haven't given up on it, as they said they wouldn't, but expect future updates to be more along the lines of bug fixes than big additions. The mod won't see Xen, for starters. The older version of the Source engine the mod uses isn't up to handling the pretty Xen they want to make, see, and they can't change it.
A few Collectiveers took to Reddit on Sunday in a 'Please Ask Us Questions, Thank You' session.
Asked whether Xen would ever be added to the free mod, level designer Joe R replied, "It's impossible. In order to make Xen look the best it possibly can look, we need to upgrade the engine with new features. These new features will make it incompatible with the mod version."
Fellow level designer Jordan F added, "Organic environments are not something the Source engine does particularly well. Making Xen look as good as we want it to requires lots of improvements to the way the engine handles rendering and lighting. We can't port these changes back to the mod because we don't have source code access to that engine version."
As for whether features already added to the new version will come back to the mod, Joe R explained: "Since the paid version is on a newer engine, we aren't able to back-update the mod version with some of the features of the paid version. That being said, we are looking into porting bug fixes onto the mod version that are found in the paid version."
And to round up a few other questions, yes they are working to a schedule but no they won't tell you it, and Mac and Linux versions are in the works.
Anyway, do go read Marsh Davies's take on Black Mesa if you haven't already.