Reason:
mediawiki/skins/CustomPage seems to have been a one off experiment by @tosfos and hasn't received any code since 2016. It also lacks resource files such as screen.less ( T154803 ).
- Archive documentation on mediawiki.org ( https://www.mediawiki.org/wiki/Skin:CustomPage )
- Remove from mediawiki/skins (along with in .gitmodules)
- Remove tests in integration/config (zuul/layout.yaml)
- Remove from translatewiki (groups/MediaWiki/mediawiki-skins.txt)
- Empty repository and add a OBSOLETE file.
- Archive the extension's diffusion repository: rSCUP
- Mark the repository read-only in Gerrit: https://gerrit.wikimedia.org/r/#/admin/projects/mediawiki/skins/CustomPage
- Delete the GitHub mirror: https://github.com/wikimedia/mediawiki-skins-CustomPage