Let users filter plugins that are no longer supported. I'd imagine something like this: Developers have to supply specific versions in their metadata like
Compatibilitylist: 2.6.0 2.6.1 (and so on) 2.7.1
Users could then mark a checkbox "Only show plugins compatible with the current version of wordpress".
The WordPress-Internal search might filter the incompatible one automatically.
Of course, we would see many updates to plugins which would only be used to "bump" the version, but then again you could easily see which plugins are actively maintained and which not.