Go to Special:AbuseFilter/history and click "User" to make the table sorted by user. The page is refreshed and the url is updated with sort=afh_user_text but nothing changes, the table is still sorted by timestamp. Now click "User" again and the order is changed from descending to ascending but it's still sorted by timestamp.
Per @Anomie's comment on https://gerrit.wikimedia.org/r/#/q/Ic755526d5f989c4a66b1d37527cda235f61cb437, this sorting would turn out to be very slow (if it worked) during the field's migration (T188180), so I removed it. This task should track the removal and thoughts on reimplementing (if possible or necessary).