https://lists.debian.org/debian-backports/2019/03/msg00043.html
Confirms the Jessie backports archive. I have just dropped it from the sources.list
Thanks Bernie for your feedback.
It seems we've fallen into the same confusion due to both the
backports repository being moved to the archive and the jessie-updates
repository being deleted all at once.
I didn't saw your link at first but it gives reals clues about the
situation. Especially [1]. I should have subscribed to the backports
list. I thought theses packages was also included in the LTS. I'm now
going to take a stretch ! Anyways, it now perfectly explains why this
repository stand for expired. It is indeed expired ! Very well
engineered to require us to by-pass the warning in order not to be
taken in default. It let us the full power to do what is required but
in an explicit way. Both of the best world. I'm left with work to do,
but once again as an happy user of Debian. ;)
Pierre
[1] https://lists.debian.org/debian-backports-announce/2018/07/msg00000.html