Valve are still updating Team Fortress 2 (TF2), and a bigger technical update is currently in the works that will bring 64bit and Vulkan support to the Linux version.
the only reason why i think they do is because they had recently shut down the source 2 tf2 port project, but kept other fan projects like portal 2 prequel and oked it. makes it seem like they have something in store for team fortress, albeit the port job was illegally moving assets into a different development ground.
I think that it's just on life support, which is fine. Many companies would have completely abandoned TF2, so Valve putting in some effort to keep it alive is nice.
A proper sequel would be very cool, but I don't think we can have such nice things.