Yep, it's one of those silly rules that Uefa have added where a player has to have been at the club for 2 years between 15-21 to be allowed on the 'b' list, which just encourages clubs to stack up 16-17 year olds in the hope that a handful will be good enough at 19-20 to fill out their squad.
On paper these ideas seem sensible but in truth, just like PSR, they encourage treating youngsters like cattle.