It appears to be literally unused since forever, and that it's defined as NOT NULL is causing issues under PostgreSQL as FanBox::addUserFan does not set a value for that colum and thus it ends up being NULL and that results in DB errors under PostgreSQL at least when trying to create a new userbox via Special:UserBoxes: Error: 23502 ERROR: null value in column "userft_order" violates not-null constraint.

(Note that PostgreSQL support for FanBoxes is not yet in master and won't land until the work on T227345 is finished.)