It looks like Valve has decided to roll out a new way for companies to offer beta programs for their games. According to SteamDB creator @thexpaw, Valve shipped the playtest button to the steam store.
This can be seen on the game, Total War: Elysium, a hearthstone-like card game. Currently, this appears to be the only game with the Playtest button.
It is assumed that the new playtest feature would be replacing the beta key program that Steam originally had. The Beta Key program was not the greatest and sometimes was just a jumbled mess for some developers. This feature will give developers direct access to approve or deny a playtest request, making it a lot easier to manage for both the developer and the requestee.
I wonder when Valve will start rolling this out to more games in Beta?