Problem: Link to not existing page in the same wiki is marked red. But link to not existing page on Commons, Wikisource etc. is not red and there is only one way how to recognize it - click on it..
Who would benefit: All editors adding links to other wikis (like {{commonscat}}), readers, maintenace editors.
Proposed solution: Now exists script, which marks links to pages without Wikidata item. Maybe somethink like that would serve to this request. Another possibility is checking all these links during page rendering, usually there is not more than one or two per page. This link then can have eg. class="dead-wikisource-link" in HTML.
Doing this might be a performance concern, as each wiki linked to would require opening a separate database connection to that wiki's database. There might also be edge cases of pages that don't exist directly in the database but "exist" thanks to an extension, language variants, or something like that. And, of course, it would only work for local wikis unless you went one step worse and did an API query like third-party wikis do when using Commons images. Anomie (talk) 15:01, 15 November 2017 (UTC)[reply]
Support. More generally, we have a problematic case of links between two projects that become dead without notice. For instance, a Wikipedia article has a link to Wikisource like s:something, then this page is renamed to s:something else, then someone deletes redirect as it is not used on Wikisource, and a link from Wikipedia becomes dead. We should be able to track this at least within our projects — NickK (talk) 17:08, 11 December 2017 (UTC)[reply]