For most old Wikimedia-maintained skins, the maintenance commits are largely automated build and translation changes, though they do still keep them kind of viable for modern features like notifications, or major architectural changes.
For example, MediaWiki 1.35 added Mustache templating: https://www.mediawiki.org/wiki/Manual:How_to_make_a_MediaWik...
So here's the commit to MonoBook converting it to use Mustache: https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/ski...