That was 2019. Even the people who worked on that left
I don't think that's entirely true either, a good number sure, but most of the old guard like most people think of didn't even work on Alyx to begin with.
As far as I can find, you actually still have a good number of the old guard left. Robin Walker (directed Alyx) is still there, some of the level designers like Aaron Barber, Dave Riller and Randy Lundeen. On the technical and art side you still guys like Dhabih Eng, Jay Stelly, and Tristan Reidford.
Not to mention, assuming HLX did start development after HLA in about 2020-2021, you have overlap from people like Greg Coomer, Jeep Barnet, and the chad Dario Casali.
Did a bunch of old hl1 and hl2 devs not come back a couple years ago? Valve also went on a hiring spree and grabbed a bunch of the level designers that worked on Titanfall 2.
There was a period of mass hiring yeah (see this
miro board and look under the Employee Resumes section to the right)
One of the highlights is they got a guy who worked on Forza (iirc) to do a bunch of advanced stuff with vehicles. They also got a guy who worked on a Half-Life Alyx mod as well, like they did with the guy who made Minerva: Metastasis.
As for the old guard returning, I can't find anything that proves that, unfortunately. I think that all started because people saw all of the clips from the documentaries where the developers reminisce about making HL1 and 2. I really,
really hope that it is true that a lot of them are returning for one last hoorah, but until we have concrete proof of that, it's only a rumor.