Steam Deck will now allow you to download games with the screen off

Valve has started testing a new low-power download mode for the Steam Deck that lets the handheld finish game and update downloads with the screen turned off. Until now, players had to keep the console fully awake during downloads, which was annoying for large games and not ideal for OLED models due to static UI elements.
The new display-off low-power mode kicks in while downloads are active. Pressing the Power button during a download now brings up a dialog asking if you want to keep downloading with the screen off. You can also just leave the Deck idle with an active download and it will automatically switch to this mode after a timeout. Once the queue is done, the system goes to sleep.
When this feature is active on battery power, Steam Deck will automatically go to full sleep mode if the battery level falls below 20%.
— Valve
While downloading with the screen off, pressing a button or moving the Deck briefly shows a simple status screen with progress information. From there you can either wake the system fully or let it continue with the screen off again.
For Steam Deck OLED owners, this is more than just convenience. Reducing the time the full UI sits on screen during long downloads lowers the chance of burn-in, while still keeping the game library up to date in the background.
Source: Valve