the posted video is for using a community made add-on. its not a core piece of blender code that has been removed.
and as far as the addon goes, “gone forever” is an excessively dramatic statement. it would just need to be updated for changes in newer versions of blender.
it looked like the author was working on that at least for the 2.8 series, as discussed in the addon’s thread here:
blender storing the reference background image in the camera or viewport shouldn’t really impact the feasibility of this add-on, either. you’re calculating perspective from worldspace axis coordinates versus camera projection coordinates. there’s already a python utility function that can handle that conversion.