Hello!
Some months ago I asked the question here: Should externally available (and partially non-free) resources result an anti-feature?
And recently I found that description for this auti-feature is a bit incorrect, or at least, its Russian translation (I checked right now English version, and I see it uses generic non-free term. Russian uses “closed-source additions” term). I think, I should clarify this via WebLate…
In my case, additions (game assets packs) for TheXTech are impossible to be closed-source as they work in a plain open-source form and can be alternated in any way. The only reason why they are non-free because of their license - they were originally made by community without care about proper licensing, and a lot of copyrighted materials (sprites, sound effects and music taken/adjusted from commercial games) were used. So, their license is basically non-commercial use only, and in addition, they give some legal risks at some countries like USA.
EDIT: Just now I found that at Weblate the correct term now used, but seems the site has outdatet and incorrect variant…
EDIT2: However, I sent suggestions to some other items at WebLate.