Hi,
In the RfC meeting for the Future of magic links RfC, it was agreed upon to disable magic links for the MW 1.28 release by default and begin the deprecation for Wikimedia sites. One of the subpoints was to provide migration tools and similar functionality. For RFC and PMID links, they were added to the default core interwiki map, and the Wikimedia one. But for ISBN, it points to the local wiki, so an interwiki link doesn't make sense.
The two main proposals are:
Virtual namespace ([[ISBN:0-7475-3269-9]]), which is similar to existing link syntax (double brackets), but is also a bit magical since there haven't been any new virtual namespaces since Special and Media in the initial version of MW.
Parser function ({{#isbn:0-7475-3269-9}}), which is typically how new syntax/functions are added, but is farther away from traditional link syntax.
I would appreciate comments on https://phabricator.wikimedia.org/T148274 - I've written a patch for the virtual namespace as a POC, but writing one for the parser function if people prefer that, should be trivial.
-- Legoktm