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
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Thank you for providing the links to the Ckeditor 5 problem when using the gin theme that may occur after updating Backdrop to the 1.32.0 release, and the link to the new release of gin that...
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Backdrop CMS 1.32.0 Upgrade Issues or Feedback
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Add blocks filtering problem