Manual:$wgEnableMagicLinks

Controls whether help>Special:MyLanguage/Help:Magic links|magic link functionality should be enabled.

As of MediaWiki 1.28, magic links are now disabled by default.

It has been proposed to remove magic link functionality from MediaWiki in a future release, if you depend upon or use them it is requested that you comment at Requests for comment/Future of magic links.

Example
For example, to enable ISBN auto-linking, you can use:

If auto-linking was previously disabled and you've enabled it as described above, you'll most likely need to man>Special:MyLanguage/Manual:Purge|purge the cache of the affected pages.

For purging all pages in a given namespace or all pages in all namespaces, you can use the  maintenance script.