Having this around and not throwing hard deprecation notices is confusing. This has been deprecated since 1.32
https://codesearch.wmcloud.org/search/?q=setupSkinUserCss&i=nope&files=&repos=
- Fix Modern https://gerrit.wikimedia.org/r/c/618498
- Fix Timeless https://gerrit.wikimedia.org/r/618503
- Fix CologneBlue https://gerrit.wikimedia.org/r/c/mediawiki/skins/CologneBlue/+/619142
- Fix Nostalgia https://gerrit.wikimedia.org/r/c/mediawiki/skins/Nostalgia/+/619143
- Update/remove core usages and make setupSkinUserCss throw a warning if referenced https://gerrit.wikimedia.org/r/c/mediawiki/core/+/619144