This is to inform the core team that most non-WMF-maintained skins are broken since 1.36-wmf.5 to the extent of not loading any stylesheets. As these skins are in use by many operators, this unintended change would cause serious disruption. The Stable interface policy https://www.mediawiki.org/wiki/Stable_interface_policy#Removal is supposed to prevent this, but this error went unnoticed.
I've submitted the necessary patch to prevent this: https://gerrit.wikimedia.org/r/c/mediawiki/core/+/622254 https://phabricator.wikimedia.org/T266735
Demian