However, according to most rabbinic authorities, there is still a mitzvah derabbanan -- to keep the shemita with all its requirements because the rabbis have decreed that we should continue the biblical practice, although there are some who consider that there is not even a rabbinic requirement to cancel debts. See Ramo, Choshen Mishpat 67:1 and Aruch HaShulchan, ibid, note 1.

Even though only a minority of authorities consider that shemitat kesafim is no longer in practice, that minority opinion has been relied upon by many, who therefore took no steps to assure that debts were not automatically cancelled (by writing a pruzbui). The Rosh (14th century) writes in strong protest of what apparently was the practice in his society, that people did continue collecting their loans, but he does not seem to have been successful in stopping the practice. However, the Ramo and the author of Shulchan Aruch HaRav do state that although one may rely on the lenient opinions, a truly pious person should write a pruzbul to cover his loans.

The prevailing and accepted position today among rabbinic authorities is that shemitat kesafim, cancellation of debts, does apply today on a rabbinic level. Therefore, if one does not want his loans to be cancelled, he must write a pruzbul.

In his Shearim Metzuyanim Behalacha, part 3, 128:25, Rav Braun cites the Chazon Ish who reasoned that since the Gemara wrote that a pruzbul can even be written on chot hamoed, of necessity it must have meant during the sixth year.