closed_wikis where added to Family.langs property because Wikibase items may contain links to closed sites and the and newly implemented badges for sitelinks may fail due to closed wikis.

But wikimedia_sites wrongly includes obsolete sites because language_by_size property is a (mutable) list.

Family.langs class property takes the cls.codes and appends closed wikis to it. But cls.codes is a class property too which returns languages_by_size. Due to the fact that this is a mutable, appending closed wikis in Family.langs it just appends it to languages_by_size as an unwanted side effect.