Players may only be transferred during transfer windows that are set by the Football Association. The two transfer windows run from the last day of the season to 31 August and from 31 December to 31 January. Player registrations cannot be exchanged outside these windows except under specific licence from the FA, usually on an emergency basis. As of the 2010–11 season, the Premier League introduced new rules mandating that each club must register a maximum 25-man squad of players aged over 21, with the squad list only allowed to be changed in transfer windows or in exceptional circumstances. This was to enable the 'home grown' rule to be enacted, whereby the League would also from 2010 require at least 8 of the named 25 man squad to be made up of 'home-grown players'.

Answer this question, if possible (if impossible, reply "unanswerable"): In which season was it mandated that each club must have a maximum 21-man squad of players over 25?
unanswerable