This report is a followup of bug 7817 which deals with removing internal item field permanent_location from the MARC structure. Unfortunately, there is some history in Koha on using paidfor in the MARC structure as local note etc.
The paidfor field was historically used for the internal item note (952$y in MARC21) for some time in the default frameworks. I think we cannot be sure that all libraries migrated that information into itemnotes_nonpublic by now. So we will need to be careful about removing it and possibly print a warning when there is still data in it. I am not aware of another current use of it.
Moving this to be a duplicate of bug 19919 as it also discusses the removal. *** This bug has been marked as a duplicate of bug 19919 ***