Change 1013295 had a related patch set uploaded (by Ammarpad; author: Ammarpad):

[operations/mediawiki-config@master] Set wgUploadNavigationUrl for is.wikibooks

https://gerrit.wikimedia.org/r/1013295

Change #1013295 merged by jenkins-bot:

[operations/mediawiki-config@master] Set wgUploadNavigationUrl for is.wikibooks

https://gerrit.wikimedia.org/r/1013295

Mentioned in SAL (#wikimedia-operations) [2024-03-25T08:49:47Z] <hashar@deploy1002> Started scap: Backport for [[gerrit:1013295|Set wgUploadNavigationUrl for is.wikibooks (T360431)]]

Mentioned in SAL (#wikimedia-operations) [2024-03-25T08:52:07Z] <hashar@deploy1002> ammarpad and hashar: Backport for [[gerrit:1013295|Set wgUploadNavigationUrl for is.wikibooks (T360431)]] synced to the testservers (https://wikitech.wikimedia.org/wiki/Mwdebug)

Mentioned in SAL (#wikimedia-operations) [2024-03-25T09:03:17Z] <hashar@deploy1002> Finished scap: Backport for [[gerrit:1013295|Set wgUploadNavigationUrl for is.wikibooks (T360431)]] (duration: 13m 29s)

There is something weird going on. An auto-confirmed, non-sysop can not see the link. The manual says

If $wgUploadNavigationUrl is set to a url, the link is always shown, regardless of the user's permissions.

Either that is a lie or the change was not applied correctly. It could be that I am forgetting something, but nothing is pointing me in that direction. Uploads are limited to sysops on is.wikibooks. I have a theory that only sysops can see it.

The documentation is correct, and I think the change applied is correct too.

If you use https://is.wikibooks.org/wiki/Fors%C3%AD%C3%B0a?useskin=vector&safemode=1 you should see the link. So it seems a local script is removing it.