It appears that wikidata page banner uses an (essentially) undocumented* skin api ('prebodyhtml') that is only implemented in Vector and Minerva.
*It is technically documented, but there's the strong implication it is a skin-specific hack to make VectorBeta work and not meant for general usage nor expected to be implemented by skins in general.
It uses a blacklist to use an alternative approach in all other skins that (I assume) the extension author could think of.
This is a bad approach. If nothing else, given this api basically only exists in Vector and is not documented as something skins are expected to implement, it should be using a whitelist of good skins, since basically no skins implement it.
Better would be to have some sort of class constant if the api is supported, so skins can do feature detection.
Even better would be, if the feature is actually useful and a good idea, would be to document/standardize it so that all skins are expected to implement it (What this entails in our context is debatable. At a bare minimum it should be in the official "example" skin and perhaps have a more explicit code comment in skins/SkinTemplate.php.
The original context that this came up in is that WikidataPageBanner does not seem to work in Timeless and some other skins. Arguably one could band-aid this by just adding Timeless to the blacklist, but that seems like just pushing the tech-debt down the road.