The index only exists on WMF cluster, i.e. is not part of the schema defined along with the code.
The index was dropped as mentioned in T194270, but when it happened the severe outage happened related to database issues: T195520. As a quick measure, the index has been restored.
As the index is in the production database, it would be good to give it a more meaningful name, not "tmp1".
Name suggestion: term_search_unused.