A top sports lawyer says "players can walk away" at the end of their contract on June 30th. https://www.transfermarkt.co.uk/sunderland-afc/vertragsende/verein/289/vertragsendeJahr/2020/plus/1
Where are they going to walk away to? Those out of contract players may find it a curse rather than a blessing - those lucrative golden hello's may be in short supply this year.
Once their contract is over, unless they and the club have agreed otherwise they'll stop getting paid, so it's hard to imagine them hanging around for the fun of it. It's easy to imagine short term extensions, but for every player and agent that cashes in here there will be plenty who just end up out of a job as the clubs are in financial dire straights and aren't going to retain them out of sentiment. Players and agents are easy to lambast, but clubs are just as bad to be honest.