`Skin::setupSkinUserCss()` was a step in adding the stylesheets to the generated page. Skins that used to override this method now (since 1.36-wmf.5) appear completely unstyled.
==== History
Soft deprecated in 1.32, hard deprecated in 1.36, planned to be removed in 1.37 (T257990#6336906).
It was erroneously removed in 1.36-wmf.5 ([[https://gerrit.wikimedia.org/r/c/mediawiki/core/+/619144/2/includes/OutputPage.php#b2985|patch]]), breaking non-WMF skins in violation of the [[https://www.mediawiki.org/wiki/Stable_interface_policy#Removal|Stable interface policy]].
In use by 30+ skins and extensions: https://codesearch.wmcloud.org/search/?q=setupSkinUserCss&i=nope&files=&repos=
==== Ignored steps of the Stable Interface Policy
* "Code MUST emit hard deprecation notices for at least one major MediaWiki version before being removed. It is RECOMMENDED to emit hard deprecation notices for at least two major MediaWiki versions."
* "The removal MUST also be mentioned in the relevant RELEASE-NOTES file"
* "Developers MAY email wikitech-l or mediawiki-l about the soft or hard deprecation depending on severity."
* "[...] may be necessary to make breaking changes [...] In such a case, developers MUST email wikitech-l ahead of time, explaining why deprecation is not possible or not reasonable, and providing an opportunity for affected parties to raise concerns and propose alternatives."
==== Does not apply:
* "The deprecation process may be bypassed for code that is **unused within the MediaWiki ecosystem**. [...] can be searched using the [[ https://codesearch.wmcloud.org/search/?q=setupSkinUserCss&i=nope&files=&repos= | code search tool ]]."
* "Additionally, in some rare cases, it may be necessary to make breaking changes without deprecation it in a major MediaWiki version beforehand, because the old behavior cannot reasonably be emulated. In such a case, developers MUST email wikitech-l ahead of time, explaining why deprecation is not possible or not reasonable, and providing an opportunity for affected parties to raise concerns and propose alternatives."
* "Additionally, in some rare cases, it may be necessary to make breaking changes without deprecating it in a major MediaWiki version beforehand, because the old behavior cannot reasonably be emulated."
==== See also
Formalization of the solution: {T267085}