Valve writes that this isn’t meant to replace Early Access beta testing, and can even be used alongside it, but unlike Early Access, this feature is free to use for developers and customers and doesn’t support sales. A Playtest game doesn’t include things like reviews, progress toward achievements or time played under the Steam Refund policy.
The Playtest tools are in beta themselves, and SteamDB trackers noticed the initial trials back in July. As Pavel Djundik noted then, the code for Playtest has been around since 2015, even though it’s just now launching. Of course, now Valve is facing competition on