Now if the message popped up and you could click a link to see which libraries they were talking about, that would be very helpful.
It's an option that can be switched off. Not sure what the menu is (file -> preferences?).
The popup steals screen focus even if Arduino IDE is running in the background, there is no way to ignore an update, and the UI is confusing.
I guess that would be File > Preferences > Check for updates on startup? I had thought that preference only referred to updates to the IDE, good to know, thanks!