I am unsure of the places that core uses the taxonomy_index.
However, I do know that its pretty useless unless you only care about node references to taxonomy terms.
The module taxonomy_entity_index exists to fix this problem for Drupal 7.
I would wager that since Entity API is in backdrop, you'd want this functionality.
GitHub Issue #:
103
Recent comments
Also it has no maintainer. Anybody interested to maintain?
Flood Unblock analogue
Because it doesn't have a release yet?
Flood Unblock analogue
It's already been done, https://github.com/backdrop-contrib/flood_unblock how did we miss it?
Flood Unblock analogue