Qitzur Shulchan Arukh – 180:2
The seventh year lays to rest every loan, whether loaned orally, whether loaned with a contract, and even if it is an item for which there is clear responsibility [e.g. land, immobile goods, or siginificant mobile ones, where the ownership is more obvious]. Someone who gave his friend money as a business investment, such that half is a loan and half is [retained but] leftin his charge, the half that is a loan is retired, but the half that is left in his charge is not.
Shemittas kesafim (the retirement of loans on the 7th year) is Torah law only when yovel (the jubilee year) is in practice, that is, when most Jews are living in Israel, and possibly even only if we’re back on the lands as divided by tribe. The Shulchan Arukh (CM 67:1), states that today this law is midivrei soferim, a rabbinic law endorsed by prophecy, a more binding sort of man-made leglislation. This appears to be the majority opinion, that it is rabbinic law. The Ramban (Seifer haZekhus) says it’s even Torahitic. On the other side, the Raavad says it’s a minhag chassidus, a practice of piety, not binding at all. The Rama (CM, ad loc) says that some say there is a custom not to practice this law at all, dating back to the days of the Rosh.
The question is understanding what the Rama is referring to — how can one have a custom to ignore a prohibition?
The Rosh himself explains that we started inserting an explicit clause in the loan that shemitah does not retire it.
The Gra says the custom is based on holding like the Raavad. The practice is an act of piety, and in Ashkenaz it became common not to follow it.
The Mahariq (shu”t #96) limits this custom to items that have acharyus nechasim. In contrast to what R’ Ganzfried writes here, the Mahariq rules that items which are clearly identifiable and don’t often change ownership were not included in the rabbinic legislation.