I don’t know how I could test what happens upon a stable release.
Nevertheless, I found this issue that seems very related:
So it looks like the testing images currently actually have “buster-backports” already enabled.
@Sunil @jvalleroy: But “buster-backports” (the current stable) suggests to me that it will break when bullseye becomes stable with the next release.
On the other hand, maybe exactly this codenamed backports line can be part of a solution if a different approach is used:
Instead of freedombox systems doing (unattended) upgrades immediately after the release, maybe the updates of stable freedombox systems could be triggered by a (last) backports package to the then old-stable. When installed on the new old-stable it triggers the update (incrementing the codenames from old-stable to new-stable).
And on the released-testing, that last backports package just, well shouldn’t have to do much (besides switching over that backports line) if everything else was already pointing to the proper testing, and newly stable, codename.