Example for incubatorwiki: https://www.wikidata.org/w/index.php?title=Q4847311&diff=prev&oldid=982221291
Live list of them: query.
An item was created with a link to sourceswiki (or "old Wikisource", mul.wikisource...). However, the link is not displayed anywhere on the page. (Subsequently, the item was merged to another but this page doesn't display the link either.)
By the way, should this happen at all? I must have missed that sitelinks to sourceswiki are allowed.