"Valve is working on a 'game notes' system," says SteamDB creator and noted Steam dataminer Pavel Djundik, "which will be available in the Steam library, and on the web." While "game notes" is vague enough to mean plenty of different things, the consensus expectation is a way of attaching private notes to games to keep track of things while you're playing, without having to alt-tab away or write them down on paper.
Back in the day, game manuals would often have a blank page or two for notes at the back. An equivalent that could be brought up in the Steam overlay, ideally hosting screenshots and links as well as text notes, sure would be helpful for all those games where you have to remember codes or strings of numbers. So many of my screenshots are random walls with things that might be puzzle clues on them, or dialogue scenes where an NPC gives directions.
Notes are a feature that's been requested for years, with a post on the Steam subreddit from 2018 suggesting, "This would be great for working strategies on CS and noting builds in Dota 2." Another user on the Steam forum suggested the same thing in 2019, saying, "I would love to be able to take notes within the Steam platform so I can have my gaming notes easily accessible to me on the platform I am interacting with instead of having to worry about loosing my physical gaming notes and having to find them each time I game. I believe there is a large percentage of the Steam Community who who take some form of notes while gaming, and there are a lot of games such as roguelikes or metroidvanias where note taking is almost required to make progress and that are very suitable for note taking."
I did write down a lot of theories and potentially cromulent words when I played Her Story, and used to draw dungeon maps on scrap paper when playing old RPGs that didn't have automapping, but I'd much rather have a version of that on the cloud. And one I don't have to alt-tab into another program to look at. A place to store frequently used GTA 5 cheats or Skyrim console commands, as well as all the passwords and safe combinations you find in immersive sims.
Djundik says that Valve is is "also working a notifications page, similar to the notifications page in the mobile app. The new notification system should also be used in the Steam client, once they finish replacing old vgui parts of the client (outer shell, settings, server browser, etc)."
That's a less exciting addition. I'm not a fan of the nagging of an "unread notifications" icon, and the Steam mobile app mostly just uses its notifications to remind me that games on my wishlist are on sale, which is a thing I can more easily see by looking at my wishlist.
Steam is currently having its spring sale at the moment, which runs until March 23. After that, the next Steam sale will be the puzzle fest in late April, with the big Steam summer sale beginning on June 29.