For those out of the loop, most felt Half life 3 was officially dead after one of the lead writers Marc Laidlaw leaked the plot to half life 2 episode 3 to give fans closure:

https://www.marclaidlaw.com/epistle-3/

https://www.reddit.com/r/HalfLife/comments/6vwelo/i_edited_m...

So it's surprising to see more half life being worked on!