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...
You might be interested in the sections "WE MADE IMPROVEMENTS TO BACKWARD COMPATIBILITY" and "WE IMPROVED THIRD-PARTY SUPPORT" on https://jdlrobson.com/posts/2023-01-17_the-new-wikipedia-app... which have a few more links that might be of interest.
Some of these are made a little more complicated as they involve different teams - for example the language button is owned by a team who works on a Asia/Europe timezone (compared to our US/Europe timezone).
I think you are specifically referring to https://phabricator.wikimedia.org/T325009 (still awaiting discussion) and https://phabricator.wikimedia.org/T318013 (resolved next week) ?