Manual:$wgTagStatisticsNewTable

For example in case of Special:Tags. If set to false, it will use change_tag table. Before setting it to true set $wgChangeTagsSchemaMigrationStage to MIGRATION_WRITE_BOTH and run PopulateChangeTagDef maintaince script. It's redundant when Manual:$wgChangeTagsSchemaMigrationStage is set to MIGRATION_NEW