Manual:$wgNoFollowLinks

If true, external URL links in wiki text will be given the rel="nofollow" attribute as a hint to search engines that they should not be followed for ranking purposes as they are user-supplied and thus subject to spamming. Defaults to true.

It may be desirable to configure MW to append rel="nofollow" to internal links that point to pages that haven't yet been written (so-called "red links") for various reasons that include avoiding unnecessary crawler traffic to non-extant pages or for the possibility of improved SEO by avoiding punitive action against a site's ranking due to the presence of "broken links" that aren't broken, just not yet authored.

This may be accomplished by using the HtmlPageLinkRendererEnd hook as follows:

// Add rel="nofollow" to links to pages that don't exist (redlinks)
$wgHooks['HtmlPageLinkRendererEnd'][] = 'noFollowRedLinks';
function noFollowRedLinks(
 $linkRenderer, $target, $isKnown, &$text, &$attribs, &$ret)
{
 if (!$isKnown && preg_match('/\bnew\b/S', $attribs['class'] ?? "")) {
 $attribs['rel'] = 'nofollow';
 }
 return true;
}

• $wgNoFollowDomainExceptions
• $wgNoFollowNsExceptions
• Extension:Interwiki - the easiest way to manage whitelisting (since nofollow is not applied to interwiki links)
• Manual:Nofollow
• meta:nofollow, Meta-Wiki discussion on nofollow's use on Wikimedia sites. Some of the arguments raised there may also applicable to other wikis' decisions concerning this configuration setting.

• MediaWiki configuration settings
• MediaWiki configuration settings introduced in version 1.4.0
• MediaWiki configuration settings still in use
• Parser variables
• Spam management
• Robot policy