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In 14cf8eacf1a679c08ea7df93aff17949d1a9c4df, we added the `user_search_similar_results` site setting which when enabled will use trigram matching for similarity search in `UserSearch`. However, we noted that adding the `index_users_on_username_lower_trgm` index is causing the PG planner to not use the `index_users_on_username_lower` index when the `=` operator is used against the `username_lower` column. Based on the PG mailing list discussion where support for the `=` operator in gist_trgm_ops was being considered, it stated that "I also have checked that btree_gist is preferred over pg_trgm gist index for equality search." This is however quite different from reality on our own PG clusters where the btree index is not preferred leading to significantly slower queries when the `=` operator is used. Since the pg_trgm gist index is only used for queries when the `user_search_similar_results` site setting is enabled, we decided to drop the feature instead as it is hidden and disabled by default. As such, we can consider it experiemental and drop it without deprecation. PG mailing list discussiong: https://www.postgresql.org/message-id/CAPpHfducQ0U8noyb2L3VChsyBMsc5V2Ej2whmEuxmAgHa2jVXg%40mail.gmail.com