In T304753#12119066, @Esanders wrote:

The current logic means enhancements are available on most talk pages, and can be added to more with __NEWSECTIONLINK__ (excluding the main namespace):

// Visual enhancements are only enabled on talk namespaces (T325417) ...
return $title->isTalkPage() || (
	// ... or __NEWSECTIONLINK__ (T331635) or __ARCHIVEDTALK__ (T374198) pages
	(
		static::hasPagePropCached( $title, 'newsectionlink' ) ||
		static::hasPagePropCached( $title, 'archivedtalk' )
	) &&
	// excluding the main namespace, unless it has been configured for signatures
	(
		!$title->inNamespace( NS_MAIN ) ||
		$services->getNamespaceInfo()->wantSignatures( $title->getNamespace() )
	)
);