As far as I know it’s always been the case that on blank days you can be asked to work but don’t have to (the only caveat to this is that when operating operating away from your home base, when asked to you have no choice). As you say it does not attract flexi (day off payment) so nobody (I know) does it and I can’t remember the last time Crewing asked so at home base these blanks are effectively just more days off.