Webmention is an open web standard (W3C Recommendation) for notifying a website when it is mentioned on another site.

In MediaWiki this could work in two ways:

• A Webmention could be sent when an external URL is added to a page (and when that page is updated).
• Webmentions could be received for all pages in a wiki, and displayed akin to how Special:WhatLinksHere works for local incoming links, e.g. Special:Webmentions/Lorem_ipsum would list all remote sites that link to /Lorem_ipsum. There could also be a place for people to submit URLs that the system would then check for mentions.

Some more information: https://indieweb.org/Webmention