just yesterday I was stuck for an hour because a debian package mirror went down. took a long time to talk the user through changing their sources.list so that another mirror was chosen, and the mirror chosen out of that pool was down also. finally I had to manually check for a good mirror and give them the URLs.
the user's take was "why don't they use GitHub packages?"
This is not how this works anymore. The system that is behaving this way must be relatively old at this point since almost all modern Debian based distros use the "mirror://" URI syntax now that automatically falls back to another mirror if one fails.
You are correct. While it's supported and part of the APT version in Debian, they don't make much use of it themselves, whereas most downstream distros are making use of it (eg. Ubuntu)
You can still use it in vanilla Debian, but they don't make their mirror list available easily in the correct format, so you would have to basically curl + awk the URLs into a text file and use that.
My guess is that Debian itself probably sees less than 1% of the traffic on their mirrors compared to Ubuntu and they haven't been as motivated to make this change.
the user's take was "why don't they use GitHub packages?"
"still running today" doesn't mean 100.0% uptime.