Plugins that haven't been updated in over 2 years display a notice in the WordPress Plugin Directory; why not also display a similar notice in the summary details for a plugin in the WP admin area?

That way users would be made aware of plugins that haven't been updated in a long time (when they're viewing their list of installed plugins).

Another option, to specifically address plugins that have been removed from the WordPress Plugin Directory, would be to display a notice like: "This plugin is no longer listed in the WordPress Plugin Directory. No further updates will be available for this plugin, unless it is relisted in the directory." Again, by this I mean displaying a notice in the WP admin area (installed plugins list).

Of course this would only apply to plugins that link (or linked) to WP for updates, not plugins that rely on other sources for updates.