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.
https://manpages.debian.org/bullseye/apt/apt-transport-mirro...
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.