I dont know for sure, i sign them all.
Most of the time they are good trainers, and wonderfull players.
In my maine Team i have 5 players from my academy.
In other i have 7 and 1 player from level 1
In other also 5 players.

but i think they just dispeared, because lost of people dont sign the U21 player, and i only saw 1 or 2 times a young free agent.