A theme is regarded unmaintained and removed from the Hugo Themes showcase when its demo does not build with the latest Hugo and the theme author fails to respond to our guidance (typically we give a one month time frame for author response).
We do not take third party library versions into account for removing themes from the list.
With the above said, common sense should be exercised. And I have -in at least one occasion- encouraged a theme author to update a third party library version because what was used in his theme was a few years old.