Traditionally, feathered margins are the oldest form of margin in dentistry. In the era of the belt driven handpiece, the preparations were made with a large rotating disk that would pass through the interproximal, opening the contact point and removing interproximal undercut. This was literally, the "slice" preparation. These preparations had the intent of removing undercut... thats about it.
Impression materials that could capture accurate margin detail and tissue contour were not available. Also, the bone-heads of the time, (and up to currently for those that neglect learning),believed in sinking the margin deep below the gingival tissue would place the margin in an area that bacteria could not have access. In combination with the poor impression technique, this allowed the lab technician to extrapolate the tooth contour, one or two millimeters subgingivally, and pick for themselves where the margin would go.
I, frustratingly, worked (only about two months) with a newly graduated dentist from the country of Turkey. She was recruited to a clinic because she was one of the best and brightest (she also did a one year geriatric fellowship in Pittsburg, PA). After my discussing her poor technique, she told me that is what she was taught in dental school. She added that she was also taught to prepare the occlusal reduction "flat-as-a-tabletop." She thought our "morphological tooth reduction" took too long, and took control away from the technician.
To this day, I continue to see variations of this technique done by old dentists, dentists from foreign countries, and sloppy dentists.
This margin type has uses, however, only in situations where there is not a better choice.