It looks like there's a new Valve game in the works, but don't get too excited – it appears to be a Steam quiz game that s can interact with and customise, and will likely be implemented in an Steam sale of some type. So, not exactly Half-Life 3.
As uncovered by SteamDB creator Pavel 'xPaw' Djundik on the team's official Discord (via free games on Steam, this might not be what most people had in mind.
It doesn't appear to be a simple Steam survey, as the code shows the ability to add questions and answers and to allow videos to be played whether players get a question right or wrong – just the fact that there are wrong answers immediately tracks it as a quiz game, not a survey.
As the code begins with the term "sale_section", this suggests that this quiz may appear as part of an sale – such as the Steam summer sale, which traditionally kicks off in June. As this is a feature in development, however, there's no telling for sure when it'll get implemented – or whether it'll appear at all. Still, it looks like a fun little feature if it does get added.
As for Half-Life 3, it's reportedly "not taking place" but Valve does seem to have other games in the works – other than quizzes, of course.