I ran the following as part of a Dockerfile based on the Jenkins lts image:
It started failing about 30 minutes ago (saying workflow-job could not be retrieved, after a lot of curl retries that failed with a 404). If you look at the list of files in https://updates.jenkins.io/2.121/latest/ , you can see workflow-job.hpi there. Trying to access the actual file http://archives.jenkins-ci.org/plugins/workflow-job/2.23/workflow-job.hpi fails with a 404 error.
(I wanted to flag this under website or infrastructure, but I couldn't find a matching component, which is why I filed it under core. Apologies if that's the wrong place.)