With the upcoming release of the Steam Deck, it looks like Valve is adding more quality of life improvements to Steam.
A new update has been released in the beta channel, bringing a new storage manager.
The new interface features a tabbed view listing all the drives in your system, a visual representation of what types of files are taking up storage (games, DLC, and other apps), and a list of all the games in each drive.
This is a massive improvement from the previous interface, which was literally just a list of where your library was located, how many games were installed in each drive, and how much used and free space there was.
If you want early access to the new storage manager, opt-in to the Steam beta channel by heading to Steam Settings, the Accounts tab, then toggle on Beta Participation. After a restart, you can find the storage manager either in downloads or through Steam Settings