Is this a change that's local to Wikipedia or to the entire MediaWiki open source frontend that powers it?
MediaWiki 1.38 and later bundles Vector 2022, the prior 2011 version of Vector, the pre-2011 default MonoBook, the responsive alternative Timeless, and the mobile-only skin Minerva.[2] MediaWiki admins can still set whatever skin they want to use in LocalSettings.php. $wgDefaultSkin is still vector, the 2011 version, as it has been since 1.18.[3]
1: https://wikimediafoundation.org/our-work/wikimedia-projects/
2: https://www.mediawiki.org/wiki/Bundled_extensions_and_skins