Disable large rectangle from Windows 10 Volume Overlay

You can completely disable the media OSD by using a small freeware application called HideVolumeOSD. This will prevent the OSD from appearing but will keep the media-buttons functionality (play, volume...).

On the other hand, Chromium-based browsers have started to allow the use of media-keys to control the playback. You can disable it, but it will completely remove the media-keys support: navigate to about:flags and search for media session, then disable the Media Session Service and Hardware Media Key Handling:

Some other applications may allow controlling it individually, like Spotify: Menu > Edit > Preferences > (scroll down) Display options > Show desktop overlay when using media keys

Firefox users:

I noticed all the solutions were for Chrome. Here's what to do for Firefox.

1. Open a new tab
2. Go to about:config
3. Accept the risks and continue
4. Search for thr media.hardwaremediakeys.enabled option
5. Double click it and change it to false
6. Close and re-open Firefox

That should return it to just this, without the obnoxious overlay about what is being played:

Just disable Media Session Service and it will go back to the old overlay