The Windows uninstaller offers the option "Remove settings and personal data", but that option does not actually do what it says.

It only did remove the settings from registry back when Picard was still storing the settings in registry. But since the move to the .ini file this is no longer useful.

Further more despite the settings there is more data placed in the AppData folder of the user. Picard also has a network cache there, also user specific plugins get installed there.

Caches should clearly be removed on uninstall. And I'd argue that "Remove settings and personal data" also includes plugins.