Was developed for the Vector skin long ago.
No longer used as of https://gerrit.wikimedia.org/r/540733 (T226148)
- Remove use from core and bundled or wmf-deployed repos.
- Deprecate in 1.34.
- Analyse usage in gadgets, site scripts and user scripts. Migrate, announce and/or delay removal as needed.
- Write migration guide: https://www.mediawiki.org/wiki/ResourceLoader/Migration_guide_(users)#jquery.tabIndex
- Remove in 1.35.