Won't affect the U23 at all, those players aren't on loan.
Current loanees where this comes into play are players like DC, Pjaca, Demiral, and a few other 22-25 year old randoms who didn't come through our system. Although we don't develop top players we do have a lot of HG players out on loan who form the bulk of the loanees. All that happens here is that more stay with the Primavera and then U23 squad, and you will see more short term trading where clubs can sell a player to a "loaning" team for €1m and buy him back for €1.1m the next season. It wouldn't even touch leading clubs.
Some leagues do have rules about signing underage players, so there are different ways around it depending on the age band of players.