Debian has extended support for some of their recent releases for people that are slow to upgrade. As of the time of this writing, Debian Jessie is considered LTS1. The concept of LTS on Debian is relatively new, with Jessie only being the third release to be considered as one, after Wheezy and Squeeze.
Supported must have a special meaning to Debian, because in March 2019, the people in charge of the FTP repository for Debian decided to remove jessie-updates and jessie-backports from the mirrors, causing existing Jessie installs to get errors when trying to update to fix any pending security issues that have been patched. The reasoning for this was that jessie-updates and jessie-backports were no longer getting updates and were already copied in the archive.debian.org repository, and only the security repository . jessie-updates is a bit of a strange case, it seems to have to do with the point releases (8.10, 8.11) of Debian, and once those are released anything from jessie-updates gets merged into the standard jessie repository.
jessie-backports is a bit more complicated, since although the repository is archived on archive.debian.org, it expired a while ago and won’t work unless you globally disable the validity checks for “valid-until” for all repositories.
Hopefully by the time Debian Stretch goes “LTS” I will no longer have any old systems running on it so I don’t have to do a bunch of maintenance on an operating system that is advertised as still being supported. I would rather be supported or not, instead of being in this gray area that Debian insists on.