Don’t you want to search your backups? Like it or not, Spotlight will still index them, even if it doesn’t tell you that it is.
Spotlight
My external drive hosting Time Machine backups cannot be ejected, so I switch it off or pull the cable. How can I stop Spotlight from indexing it?
How can I stop Spotlight from searching all my .sql files?
The nights are drawing in, so why doesn’t Apple do something to cheer us up – like add some subtle humour to El Capitan?
Why does mdls not give full image metadata?
How can I turn local Spotlight indexing and search off?
Yosemite’s extended search reaches out to Bing, Maps, and more. But how can you get the best out of its local search facility?
The Eclectic Light Company 