It would be useful to show when a newer plugin release exists, even if it is not available for the current Jenkins release, usually due to update site tiering.

update-center.json would need to be extended to include metadata for every plugin (or every plugin not being offered in its newest release), which is the latest release. Something like "latest":"1.2.3" should do it.

Jenkins could then show it:

• Plugin manager, updates tab:
  • If there's another update for a plugin, show an inline note, something like "The latest available release is 1.2.3 but is not available for your instance. This usually happens when a plugin requires a newer version of Jenkins to run"
  • If the plugin is otherwise up to date, a new, non-actionable table entry informing about the unavailable update
• Plugin manager, available tab:
  • show an inline note

Could even be an admin monitor, but that's probably overdoing it.