Left 4 Dead 2 is 11 years old, but Valve still keeps an eye on things, occasionally pushing out security updates and bug fixes. The Last Stand is neither of those things. It's a "collaborative effort by the community, for the community," according to the teaser, but it will be released as an official update (opens in new tab).
Valve and its creators haven't detailed what exactly the update will contain, but at the very least it looks like it will introduce a survival map from the original Left 4 Dead, also called The Last Stand, which was never officially ported over. That's the location being shown off in the teaser.
I suspect there will be more to it than that, however. There are already multiple mods (opens in new tab) that bring The Last Stand to Left 4 Dead 2, so why would Valve suddenly release a proper update after all these years for another one?
Right now, all we've got to go on is this vague introduction:
It has been many years since the infection first hit. Radio silence, no sign of life, nothing but lingering hopes... CEDA is not going to save us. But there is hope! A few brave souls have continued the fight against all odds, and soon we can all benefit from their resilience.
The Last Stand hasn't been dated yet, but it's coming soon.