On jessie hosts apt-get update is failing to download some indices due to 404 Not Found, in particular:
jessie-updates/main Sources jessie-updates/contrib Sources jessie-updates/non-free Sources jessie-updates/main amd64 Packages jessie-updates/contrib amd64 Packages jessie-updates/non-free amd64 Packages jessie-backports/main Sources jessie-backports/contrib Sources jessie-backports/non-free Sources jessie-backports/main amd64 Packages jessie-backports/contrib amd64 Packages jessie-backports/non-free amd64 Packages
apt-get update ends with the usual error:
E: Some index files failed to download. They have been ignored, or old ones used instead.
This seems to be due because Debian removed those upstream, from the topic in #debian:
Oldstable: Debian Jessie, jessie-updates and jessie-backports REMOVED 2019-03-24
A way to keep track of them is to look at the list of hosts in Debmonitor and sort by Last Update (although it might include some additional few "broken" host):