Valve is apparently working on some sort of "game notes" system, and although details are vague, it could give players some way of adding private notes without leaving the game. The information was spotted by SteamDB Creator and dataminer, Pavel Djundik, suggesting such game notes "will be available in the Steam library, and on the web." Previously, the same source suggested that Valve is working on a new notification page as well.

The new "game notes" could easily be anything, but since a lot of players have been asking for a similar feature over at the Steam subreddit, it could suggest that Valve is actually listening to the community. As noted, Valve is also working on a notification page that could be similar to the notification page in the mobile app. Valve is expected to add the notification system as soon as it replaces some of the old parts of the Steam client. Of course, you should wait to hear an official announcement from Valve, as it could be coming soon.