This summer, folks spotted a new playtesting button on the Steam store page for Total War: Elysium, that hadn't been mentioned by Valve. Well, now they've officially released details about the new playtesting feature that developers can use as a new way to run limited-time testing periods without juggling special Steam keys. The feature is still in beta, they say, so it may yet change before they officially release it. Total War: Elysium is indeed running that playtest though, if it suits your fancy.
Valve say that the playtest function is meant to standardise the approach for developers who want to give a group of players temporary access to an early version of the game. They've created tools to help developers admit a certain number of applicants along with setting start and end times for a test. They can also manually invite players if needed. For players, it's as easy as pressing the "request access" button on a game's store page (if a playtest is available) and waiting for access.
In the past, developers have at times handed out specific keys that are used to unlock a beta version of the game or posted a demo version as a stand-in for an open beta. You won't need to muck about with all that in the future, as a playtest can be managed and joined within Steam. Valve say that this feature won't be replacing the option for release-override keys or demos, so developers can still use those methods if they prefer. Playtesting won't support monetization like Early Access, so it won't be replacing that either. Developers could choose to use both features simultaneously, even.
It does sound pretty painless, though I admit I'm having a chuckle at the absolute wall of blocks and banners that will appear on the store page for an early access game with a demo version that's also running a playtest. Golly I know there's got to be an "about this game" section down here somewhere!
Here are the rest of Valve's notes on the playtest feature. For the developers in the room, you'll find the more detailed version for your use in Steam's documentation. It's still in beta, so developers who want to use the feature at present will need to contact Valve.