Example for incubatorwiki: https://www.wikidata.org/w/index.php?title=Q4847311&diff=prev&oldid=982221291
Live list of them: query.
Previous description:
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.
Hm, yeah, I also thought that wasn’t supposed to be supported, but the link exists in the entity JSON and Special:GoToLinkedPage follows it.
To me adding sitelink for these sites that with arbitrary access support but without sitelinks-on-Wikidata-items support is possible via either Chrome or Firefox, by dragging "Add links" on the left (below "In other languages"), typing title as label and click Enter Create, so I've created Q62099559
After that, I merged this item to Q131180, then history mentioned the link I merged to target item: https://www.wikidata.org/w/index.php?title=Q131180&type=revision&diff=887792943&oldid=878839260
The json codes of Q62099559 were:
{"entities":{"Q62099559":{"pageid":61922752,"ns":0,"title":"Q62099559","lastrevid":887792425,"modified":"2019-03-19T11:25:14Z","type":"item","id":"Q62099559","labels":{"en":{"language":"en","value":"Epistola I a Timotheo Capitulo 1"}},"descriptions":[],"aliases":[],"claims":[],"sitelinks":{"sourceswiki":{"site":"sourceswiki","title":"Epistola I a Timotheo Capitulo 1","badges":[],"url":"https://wikisource.org/wiki/Epistola_I_a_Timotheo_Capitulo_1"}}}}}Same for incubatorwiki: https://www.wikidata.org/w/index.php?title=Q4847311&diff=prev&oldid=982221291
The current behaviour tends to cause problems and mislead users, see
https://wikisource.org/w/index.php?title=Special:Permalink/765318#Wikipedia_link_from_the_main_page_of_the_Wikisource
(caused by https://www.wikidata.org/w/index.php?title=Special:Diff/1048313808/1054175884 and https://www.wikidata.org/w/index.php?title=Special:Diff/1042629122/1054175881 )
IMO, the "Add links" link in the left pane should be disabled in sourceswiki until T138332 is resolved.
(maybe, it should be disabled in all Wikisources, but this is another issue)