In the link you provided, under requirements you would see thatThanks for sharing that, have submitted my app for the first time among the March 2019 applicants
But there was never any requirement to stamp on the back anyways, correct?
See link below for CIC photo specifications. It doesn't mention anything about stamping the back
https://www.canada.ca/en/immigration-refugees-citizenship/services/application/application-forms-guides/citizenship-application-photograph-specifications.html
- Photographs must be printed on quality photographic paper.
- Provide the name of the photographer or the studio, the studio address and the date the photos were taken on the back of the photos
- Print the name of the person on the back of the photos.
This could be hand written or stamp by the photographer.