Due to T301247 it was possible to create sitelinks on Wikidata to all existing sites, even if they aren't supported for sitelinks (for example incubatorwiki).
We should look for all such site links and remove them, as they can cause problems for data re-user and editors (these site links are not visible in the UI and can't easily be removed via the API). These site links can also cause further problems, like T284211, both in our software and for third parties using Wikidata's data.