A patent filed in March 2020 by Valve, which was recently uncovered by SteamDB creator Pavel Djundik on Twitter, shows that Valve is looking to implement a system that lets a Steam user play a game as it downloads with what the company calls "instant play."
The way it works is that Steam will be able to track read operations of an executable file of any game. Steam will use this data to draw a digital map to prioritize specific game files during the download process, letting the player start playing the game without needing the download to finish.
In theory, this means you could start up Deathloop right when you purchase it. Steam will pull the correct files from multiple clients that will download as much of the game needed to start playing and, hopefully, depending on your internet connection, without interruption.
Here's how Valve is described its tech from its patent filing:
"Client machines running game executables of a video game(s) may utilize a file system proxy component that is configured to track read operations made by the game executable during a game session, to generate access data based on the tracked read operations, and to report the access data to a remote system. This telemetry approach allows the remote system to collect access data reported by multiple client machines, to catalog the access data according to client system configuration, and to analyze the access data to generate data that is usable by client machines to implement various game-related features including, without limitation, 'instant play' of video games, discarding of unused blocks of game data to free up local memory resources, and/or local prefetching of game data for reducing latency during gameplay."
Another added benefit from this tech is that it will free up space by using "client-side discarding and prefetching" of game data without messing with the integrity of the game and will theoretically reduce latency as you play.
If you use Origin, Battle.net, or a recent console, then you're probably familiar with the concept of playing games as they download. However, Valve's patent seems like it may be more seamless not just restrict you to a tutorial stage or just leave at the game's main menu, which isn't really 'playing as you download' then it is more just 'waiting in a game's menu as it downloads.'
Since this is just a patent filing, there's no telling if or even when instant play will see the light of the day. But knowing Valve, it'll probably come in a surprise Steam update on a random Tuesday.