Regarding the task title as written ("WMF wikis don't redirect to short url"), this is part of MediaWiki. It is not specific to WMF wikis. As far as I know, there is no use case or system documentation that requires a redirect to work. What we do have, is a rich history of problems that happen if you redirect URLs:

• T105265: Redirecting ~ -> %7E causes a redirect loop in chrome
• T106793: Pages with single quote in title are inaccessible by some clients (redirect loop)
• T67402: URLs for the same title without extra query parameters should have the same canonical link
• T100782: http://en.wikipedia.org//wiki/Main_Page raise "Redirect loop detected!" because of the double slash
• .. other tasks and commits mentioned from these
• implementation in ActionEntryPoint.php#tryNormaliseRedirect

We have, for cosmetic reasons and minor improvements in caching, carved out a safe and narrow subset of certain pageview URLs. For example, /wiki/Main Page (with a space) redirects to /wiki/Main_Page. You can find this and other examples in the test cases at ActionEntryPointTest::provideTryNormaliseRedirect.

It is expected that encountering a non-canonical "long" URL for a pageview is rare, because we never generate them ourselves (all blue links and search results point to the canonical "short" form). And just in case a search engine accidentally finds a long URL created by hand, we include <link rel=canonical> in the HTML, which is similar to a redirect, and tells the search engine to index the canonical "short" URL instead, so that the non-standard URL will not show in search results.

Regarding the task description example:

go to https://en.wikipedia.org/wiki/Wikipedia:Don%27t_delete_the_main_page
click on edit button
you get taken to https://en.wikipedia.org/w/index.php?title=Wikipedia:Don%27t_delete_the_main_page&action=edit

What should have happened instead?:
you get taken to https://en.wikipedia.org/wiki/Wikipedia:Don%27t_delete_the_main_page?action=edit

This is about the edit link, not the pageview. If the edit link should use a short URL, then it would point there directly. Generating long link, but then redirecting afterward, would be inefficient. You can learn why we intentionally prefer long URLs for actions that are not pageviews, at T276737#11838460.

I'll decline this task because the title and description both propose an outcome that is incompatible with our design requirements. Without a concrete problem, there is no action to be taken here.

If you find that part of the MediaWiki software gives you a long URL for a pageview, instead of a short URL, then that is a bug, and we will fix that. Please file a separate task if you encounter that.