The Reference Desk/UNIT Black Archives
In most of the pages of the Companions of the Doctor, the wiki states "At some point, X was taken to the Black Archive by UNIT to have his/her record as a companion of the Doctor taken. His/Her memories of the visit were subsequently erased and he/she was sent on her way. (TV: The Day of the Doctor).
While looking through the Companion pages and finding this, I think it would be very unlikely that every companion would have wound up at the Black Archive to have their memories erased. For example; Jamie wound up back in Scotland. Romana I regenerated, Zoe was sent back to the wheel.
Just looking for other opinions and to start a discussion. Thanks