1) Highlight installed games. Yes, a filter for it exists already, but it's kinda unconvinient. It's really hard to orientate in library without knowing what games you have in a computer.
2) Newsfeed. It would be great to see MMO's updates and DLC releases straight from a library.

I think a news feed would be great, as long as we could opt in/out of certain games as we please. If it gave me news for every game I owned, I wouldn't care. I don't think Discord filters out anything and so I just turned the function off.

I hope it can be completely disabled , i hate it in the new Steam UI

Don't know if its an acceptable solution:
go into your library, click on the "two sliders" icon, choose "customize grid view", activate the "status" checkbox. Now you will see a small icon (looks like a computer display) below the cover if a game is installed

Oh, it's working. But I prefer list view over grid. Anyway, thank you.