Requests for comment/Future of magic links

Background
Magic links are a feature of MediaWiki core that create automatic links for 3 hardcoded external identifiers:


 * -> ISBN 0-7475-3269-9
 * -> PMID 1234
 * -> RFC 1234

For the purposes of this RfC, we are not considering free external links (e.g. typing just https://www.example.org) to be a magic link.

Problem
These are hardcoded, inflexible, un-localizable, and generally unexpected. If this feature were proposed today, it would be rejected in favor of using templates or interwiki links. There have been long standing requests to make them disable-able, move them to an extension, or remove them outright.

Proposal
As of 309528 it is now possible to disable magic link functionality using. Our eventual goal should be to remove all magic link functionality from MediaWiki core. While moving them to an extension would allow us to remove them from core, it would probably entrench magic links even deeper in the parser as we would need a mechanism and hook system to support an extension to add magic links.

Instead, we should:
 * Add a tracking category to any page that uses a magic link (3 categories, one for each link type)
 * In the MediaWiki 1.28 release, default magic links to being disabled. Wikimedia wikis would still have it enabled for now.
 * Encourage users to migrate to using templates (e.g. w:Template:ISBN) or interwiki links, probably using bots and other assisted tools.
 * Remove magic link capabilities in MediaWiki 1.30 (1 year later, also LTS).
 * We would retain  and co. but they will always return false, in order to signal to extensions that the magic links are disabled.
 * Old revisions would no longer have autolinks, but that should not reduce the readability of the content as they are well known identifiers.
 * Move Special:Booksources to an extension.