Per https://www.mediawiki.org/wiki/Extension:ShortUrl

If you're here to enable the extension for a Wikimedia Wiki, then you shouldn't process the request. Extension:UrlShortener should be used instead

Discoverability of this is really bad, but https://w.wiki/ will take you to the special page on metawiki (https://meta.wikimedia.org/wiki/Special:UrlShortener) where any URL for a Wikimedia content project can be shortened to something like https://w.wiki/yM

In T286142#7276611, @bd808 wrote:

Discoverability of this is really bad, but https://w.wiki/ will take you to the special page on metawiki (https://meta.wikimedia.org/wiki/Special:UrlShortener) where any URL for a Wikimedia content project can be shortened to something like https://w.wiki/yM

T288653: Make URLShortener service discoverable for users