Checking all releases with the following script it seems to me that updates from appstream to ftp-master are done on "testing" and "unstable" only leaving out "stable" notably since the beginning in 2016. Is there some rationale behind this or is it a bug? package appstream has hard reverse dependencies for GNOME and others that rely on it to get the *updated* AppStream-Data for them to use.
$ cat appstream-check.sh #!/bin/bash WGET='wget --quiet --show-progress --timestamping' for RELEASE in sid buzz rex bo hamm slink potato woody sarge etch lenny squeeze wheezy jessie stretch buster bullseye bookworm trixie; do for COMPONENT in main contrib non-free; do ${WGET} "https://appstream.debian.org/data/${RELEASE}/${COMPONENT}/Components-amd64.yml.gz" -O "${RELEASE}-${COMPONENT}-appstream-Components-amd64.yml.gz" ${WGET} "https://ftp.debian.org/debian/dists/${RELEASE}/${COMPONENT}/dep11/Components-amd64.yml.gz" -O "${RELEASE}-${COMPONENT}-mirrorcpy-Components-amd64.yml.gz" done done stat --format '%y %n X%sX' *.yml.gz | grep -v 'X0X' | cut -d' ' -f1,4 | column --table | sort $ ./appstream-check.sh [...] 2017-06-16 stretch-contrib-mirrorcpy-Components-amd64.yml.gz 2017-06-16 stretch-main-mirrorcpy-Components-amd64.yml.gz 2017-06-16 stretch-non-free-mirrorcpy-Components-amd64.yml.gz 2017-06-18 stretch-contrib-appstream-Components-amd64.yml.gz 2017-06-18 stretch-main-appstream-Components-amd64.yml.gz 2017-06-18 stretch-non-free-appstream-Components-amd64.yml.gz 2019-06-24 buster-non-free-appstream-Components-amd64.yml.gz 2019-06-24 buster-non-free-mirrorcpy-Components-amd64.yml.gz 2019-06-27 buster-contrib-appstream-Components-amd64.yml.gz 2019-06-27 buster-contrib-mirrorcpy-Components-amd64.yml.gz 2019-07-03 buster-main-appstream-Components-amd64.yml.gz 2019-07-03 buster-main-mirrorcpy-Components-amd64.yml.gz 2021-08-01 bullseye-contrib-mirrorcpy-Components-amd64.yml.gz 2021-08-08 bullseye-main-mirrorcpy-Components-amd64.yml.gz 2021-08-08 bullseye-non-free-mirrorcpy-Components-amd64.yml.gz 2021-09-11 bullseye-non-free-appstream-Components-amd64.yml.gz 2021-10-09 bullseye-contrib-appstream-Components-amd64.yml.gz 2021-12-18 bullseye-main-appstream-Components-amd64.yml.gz 2021-12-31 bookworm-contrib-appstream-Components-amd64.yml.gz 2021-12-31 bookworm-contrib-mirrorcpy-Components-amd64.yml.gz 2021-12-31 bookworm-non-free-appstream-Components-amd64.yml.gz 2021-12-31 bookworm-non-free-mirrorcpy-Components-amd64.yml.gz 2021-12-31 sid-contrib-appstream-Components-amd64.yml.gz 2021-12-31 sid-contrib-mirrorcpy-Components-amd64.yml.gz 2022-01-01 bookworm-main-appstream-Components-amd64.yml.gz 2022-01-01 bookworm-main-mirrorcpy-Components-amd64.yml.gz 2022-01-01 sid-main-appstream-Components-amd64.yml.gz 2022-01-01 sid-main-mirrorcpy-Components-amd64.yml.gz 2022-01-01 sid-non-free-appstream-Components-amd64.yml.gz 2022-01-01 sid-non-free-mirrorcpy-Components-amd64.yml.gz