The repository jessie-backports Release does no longer have a Release file

Question

I get the following output (and error at the end) when I run apt update on my system:

$ sudo apt update
Ign:1 http://deb.debian.org/debian stretch InRelease                                                                                                                                                                                                         
Hit:2 http://packages.microsoft.com/repos/vscode stable InRelease                                                                                                                 
Ign:3 http://dl.google.com/linux/chrome/deb stable InRelease                                                                                                
Hit:4 http://security.debian.org/debian-security stable/updates InRelease
Hit:6 http://ftp.us.debian.org/debian stable-updates InRelease                                                                                              
Hit:7 http://fai-project.org/download stretch InRelease                                                              
Hit:8 http://repository.spotify.com stable InRelease                                                                 
Hit:9 http://deb.debian.org/debian-security stretch/updates InRelease                          
Hit:10 http://dl.google.com/linux/chrome/deb stable Release              
Hit:12 http://deb.debian.org/debian stretch-backports InRelease          
Hit:13 https://download.docker.com/linux/debian stretch InRelease
Hit:14 https://deb.nodesource.com/node_10.x stretch InRelease
Ign:15 http://deb.debian.org/debian stable InRelease
Ign:17 http://deb.debian.org/debian jessie-backports InRelease
Ign:18 http://deb.debian.org/debian jessie InRelease
Hit:19 http://deb.debian.org/debian testing InRelease
Hit:5 https://release.memsql.com/production/debian memsql InRelease
Hit:20 http://deb.debian.org/debian unstable InRelease
Hit:21 http://deb.debian.org/debian stretch Release
Hit:11 https://packagecloud.io/slacktechnologies/slack/debian jessie InRelease
Hit:23 http://deb.debian.org/debian stable Release  
Err:25 http://deb.debian.org/debian jessie-backports Release
  404  Not Found [IP: 151.101.248.204 80]
Hit:26 http://deb.debian.org/debian jessie Release
Reading package lists... Done
E: The repository 'http://deb.debian.org/debian jessie-backports Release' does no longer have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

I don't understand why I'm getting this error.

$ cat /etc/apt/sources.list
# These lines should work for many sites

deb http://deb.debian.org/debian stretch main contrib non-free
deb http://deb.debian.org/debian-security stretch/updates main contrib non-free
deb http://deb.debian.org/debian stretch-backports main contrib non-free

# repository that may contain newer fai packages for stretch
deb [trusted=yes] http://fai-project.org/download stretch koeln

$ grep jessie-backports /etc/apt/sources.list.d/*
/etc/apt/sources.list.d/jessie-backports.list:deb http://deb.debian.org/debian/ jessie-backports main contrib non-free
/etc/apt/sources.list.d/jessie-backports.list:# deb-src http://deb.debian.org/debian/ jessie-backports main contrib non-free
/etc/apt/sources.list.d/jessie-backports.list.save:deb http://deb.debian.org/debian/ jessie-backports main contrib non-free
/etc/apt/sources.list.d/jessie-backports.list.save:# deb-src http://deb.debian.org/debian/ jessie-backports main contrib non-free

Some system information:

$ lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description:    Debian GNU/Linux 9.8 (stretch)
Release:        9.8
Codename:       stretch

Answer 1

A Debian version has five phases in its life cycle:

• testing: the version is in development, and has no backports or security archive
• stable: the version is now released, and a backports and security archive are created
• oldstable: the next version after this one is released. The security support is still maintained by the security team for all architectures, and the backports archive is still supported, until a year after it became oldstable.
• long-term support (LTS): a year after the release of the next version of Debian, the security team stops supporting this version, and the LTS team takes over. The LTS team does not support backports, and it does not support all architectures; only a limited list of architectures is supported. Additionally, not all packages are supported during the LTS phase; notably, things like webbrowsers etc are explicitly not supported (but there are many other things).
• No longer supported: the LTS phase ends 5 years after the original release of that version; at that point, the version is moved to archive.debian.org

Jessie has become oldstable on June 17th 2017, and therefore became LTS on June 17th 2018. As such, the jessie-backports repository is now dormant. It was retained for a while to allow for certain valid use cases, but the backports administrators have recently done some maintenance work, and disabled the jessie-backports repository; that is the reason you are now getting those errors.

The backports repository is meant to contain software versions that are available in the next version of Debian; i.e., jessie-backports will contain packages with versions as shipped in stretch. As such, if you have already moved to stretch as you claim to, enabling jessie-backports on your system should provide no benefit at all, and you should just stick with the stretch repository and move on from jessie.

Answer 2

Since Jessie is no longer the most recent stable release for Debian (Stretch is) I believe they no longer support jessie-backports. It is now stretch-backports. It is not very safe to run stretch-backports on a jessie based machine since stretch-backports are recompiled based on what's available in Stretch. Unfortunately your options are:

1. upgrade to stretch
2. remove jessie-backports (which appears to be in /etc/apt/sources.list.d/)