In commit f9118dd "npp-compatible-versions" has been added in plugin list for helping not only Pllugin Admin but also Plugin Manager (Plugin Loader) to check the compatibility with the latest version of plugin (if the version of installed plugin is the latest one).

However, for Plugin Manager if the version of installed plugin is the previous one, there's no info to know if this plugin with old version is compatible with running Notepad++. In order to filter better old version plugins, "old-versions-compatibility" is added in plugin list, so Plugin Manager can use it to check the compatibility to decide load it or not.