$wgHandheldStyle, added by Brion back in 2008 (dc821ec1 / r38116), it slightly useless - it was only ever implemented for Monobook, and the only use-case given was using Chick's CSS for mobile devices with Monobook being the default skin - and now Chick is no more and Monobook isn't even the default.
Apart from that, it's only used to load styles from [[MediaWiki:Handheld.css]] if it's set (ResourceLoaderSiteModule.php).
Either it should be used consistently for all skins, preferably inside ResourceLoader, or it should be killed off. Personally I'd vote for killing.
Version: 1.22.0
Severity: minor