It looks like Valve has been quietly testing a 'Playtest' feature on Steam that would make life much easier for players and developers.
As highlighted by SteamDB creator xPaw on Twitter, Valve seems to be trialing a Playtest button that lets Steam users request access for upcoming playtests.
As it stands, Steam developers who want to let people try their projects ahead of release must hand out Beta keys. That hardly sounds like the end of the world, but the introduction of a dedicated Playtest button that devs can slap on their Steam page would massively streamline the process.
Right now, Total War: Elysium appears to be the only title on Steam that has implemented the feature (though there could be others that have evaded detection), which you can see in action right here.