If Half-Life 3 were to start development today, none of the primary writers for the series would be involved. Of that group, only Chet Faliszek remained— until today.
Faliszek ends his career at Valve today after 12 years and many fantastic titles under his belt, including both Episode DLC packs for Half-Life 2 along with both Left 4 Dead and Portal titles.
For whatever reason, Valve has been losing their top writers in the last year. Mark Laidlaw (Half-life, Half-life 2) left the company back in January of 2016 and was followed by Erik Wolpaw (Portal, Half-Life 2), who also departed this February 2017.
You can read more about Faliszek and his experience leaving Valve here.