How to really exclude Time Machine Backups from Spotlight-indexing?
According to Apple, at https://web.archive.org/web/20180101183140/https://support.apple.com/kb/PH21902?locale=en_US

If you add a Time Machine backup disk to the privacy list, you will
continue to see messages that Spotlight is indexing your backup disk.
This indexing is necessary for Time Machine to function properly and
can’t be disabled. Spotlight does exclude from searches any items you
store on your backup disk that are not part of a Time Machine backup.