From this year players who are in expiration on their contract can sign during the winter transfer window for free with a new team and they can leave their old team from the end of the season (1st July 2021).
I don't know if it is only an Italian change or a European change, till last year they could sing only after the end of the winter transfer market if they were in this situation.
Actually, I think it was always like this. A player can start negotiating with another club when he has less than 6 months remaining on his current contract.
Since 99% of the contracts expire last day of June, it gives the players a chance to start negotiating on January 1st - which is the 1st day of the winter transfer window.
A lot of good players on that list, BTW. I'm really surprised City is letting Aguero go.