Laidlaw left Valve in January 2016 because he wanted to return to writing his own original stories. Now Laidlaw is back in the spotlight as he has published a story called "Epistle 3". All names have been changed for legal reasons but it's obvious that this is (or was) the story of Episode 3. Presumably, that game is no longer under development.
Due to heavy traffic, his website is currently down but you can read a cached version over here.
Epistle 3 https://t.co/8BEG25AV7A
— Marc Laidlaw (@marc_laidlaw) August 25, 2017