Manual:$wgEnableMagicLinks

Controls whether 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.