Jay Pinkerton's name was spotted on Valve's employee list by one of Reddit's sleuths, hinting that the co-writer of Portal 2, Left 4 Dead, Left 4 Dead 2 and Team Fortress comics returned to the fold. Valve later confirmed this to be true.
News of Pinkerton's return to Valve first , as OWLverlord relayed it in a thread, after they noticed him on Valve's employee list. Pinkerton is currently listed under the "Other Experts" section of Valve's website, along with GabeN himself. That said, he is likely working on areas other than writing for Valve's upcoming games.
Most famous of Pinkerton's work for Valve has been exactly that - writing, and his most famous achievement is co-writing Portal 2 with Erik Wolpaw. Somewhat ironically, Wolpaw has also left Valve, just months before Pinkerton did, but he hasn't come back yet as he is apparently working on Psychonauts 2.
This migration happened during the so called "writer exodus" from Valve, which started after Wolpaw left the company in February 2017, followed by Chet Faliszek left the company in May 2017 and finally Pinkerton in June 2017. Marc Laidlaw, writer of Half Life, preceeded them all though, as he left the company in 2016.
While Valve's current game in development, Artifact, is a card game with barely any narrative to it, it's highly likely Pinkerton will be working on other games in development that were announced by Gabe Newell back in March 2018.
Whatever Pinkerton's job turns out to be, it would be really nice to see some of his influence in the upcoming games' lines. His and Wolpaw satire was a neat bit of spice that made everyone love GlaDOS while hating the character at the same time.