It is rarely used now days in my experience and I have never used a flat file that is over a decade old. Storage used to be expensive so this would ensure that old data got purged. Most flat files are useless after a month or less. Even if a mistake did happen, most MVS shops have pretty good backups.

Also now days, data files that haven't been accessed for some period of time usually gets sent to tape/cartridge in some automated system like this: https://en.wikipedia.org/wiki/Tape_library#/media/File:Stora...