Of course you can see the full list at /System/Library/CoreServices/backupd.bundle/Contents/Resources/StdExclusions.plist.
One of the biggest is iPhone backups from iTunes. If I were a normal user and my MacBook didn't boot up tomorrow, I'd be surprised to find my backup didn't include critical files like this. For example, apps that are no longer in the App Store but are in your backup can be restored. Once you don't have that backup anymore, they can't.
It's just one of the reasons Apple users are forced to have multiple backup services if they want reliable and complete backups.
I'm not sure why the iPhone backups on your system didn't get backed up. The device backups stored in ~/Library/Application Support/MobileSync/Backup/ is not in the exclusions file that I see.
P.S.: The latest app binary files may not get updated/synced on the Mac post iOS 9/iTunes upgrade.
I double checked my drive and you're correct with regard to mobile backups. That's reassuring.
It would be nice to have a utility to give a definitive diff of what's on your hard drive that's not on your: Time Machine drive / [Carbonite, Backblaze, etc] backup.
One of the biggest is iPhone backups from iTunes. If I were a normal user and my MacBook didn't boot up tomorrow, I'd be surprised to find my backup didn't include critical files like this. For example, apps that are no longer in the App Store but are in your backup can be restored. Once you don't have that backup anymore, they can't.
It's just one of the reasons Apple users are forced to have multiple backup services if they want reliable and complete backups.