In our LTS Jenkins we set the update center URL to https://updates.jenkins.io/stable/updates/update-center.json. Currently, this resolves to https://mirrors.updates.jenkins.io/dynamic-stable-2.492.3/updates/update-center.json. Unfortunately, this returns a 404.
We figured out that the Tool Installer seems to make use of the UC URL. For example, when trying to install Maven our Jenkins would try to get https://updates.jenkins.io/stable/updates/hudson.tasks.Maven.MavenInstaller.json, resolving to https://mirrors.updates.jenkins.io/dynamic-stable-2.492.3/updates/hudson.tasks.Maven.MavenInstaller.json, which doesn't exist right now.
Interestingly, the index at https://updates.jenkins.io/stable/updates/ aka https://updates.jenkins.io/dynamic-stable-2.492.3/updates/ does exist, but the links to the concrete files are all dead.
Since the index still shows I suspect that the missing links/files are not intended.