Sepia history.
In print processing, sepia tones came from using a stable toner of silver sulfide (sepia), instead of standard silver toner (B&W). With the more stable toner, a sepia toned photograph will survive 50% longer than a standard B&W. If properly stored, it will last 100% longer (200 years).
The sepia toner tended to make the darker areas of a B&W image easier to see details, without losing contrast. It could also be used to soften an over sharp photograph. This softening process was used for portraits.
In the end, sepia toned images are shades of brown, instead of shades of grey. They are definitely not shades of yellow. As a true sepia toned image began to fade, depending on the environment it was in, it would color shift towards yellow or red; but the detail in the image would also be diminished at that point.
Just stuff to think about, when creating modern, digital sepia images.