Sforno on Deuteronomy 15:4/ספורנו על דברים ט״ו:ד׳

1אפס כי לא יהיה בך אביון, even though I have said that you must forgive overdue debts, the fact is that there will be no need for this commandment to apply in practice as there will not be anyone in that kind of need. There can hardly be a question that during all the years when Joshua was the leader of the people this promise applied and no one was in need of having past due debts forgiven. We have it on irrefutable authority that as long as Joshua and the elders of his generation were alive the Jewish people kept the Torah in an exemplary fashion. (Joshua 24,31)