Excluding /Library (sic, not ~/Library) from Time Machine Is there anything - at all - of value I will lose as a user, by excluding /Library?

Let's assume that rebuilding from the backup to a new machine is not important.

So, what is important is absolutely not losing any of my Notes notes, email,   perhaps Calendar materiel, etc., from Apple apps.

Specifically what will be lost if I exclude /Library?

(Perhaps some settings, or? Anything at all?)
Apple and third-party applications may have a variety of subsidiary files stored in /Library.
Much of it will be in /Library/Application Support.

Third-party fonts may be in the Fonts subfolder. 

But there may also be Dictionaries, Color Profiles, LaunchAgents, Privileged Helper Tools, system-wide Preferences (including network settings), printer drivers, QuickLook and Spotlight plug-ins, AppleScript Scripting Additions, Frameworks, and so much more, depending on what you've installed. Anything for which there's a named folder!

My advice is that you probably won't save much space on a backup disk by excluding /Library, and if something goes wrong, it's easier to restore one file or one folder than have to re-install a whole load of applications and the OS just to be sure.