**Reason**: Unmaintained per https://www.mediawiki.org/wiki/Topic:Udfz610247i2zf1c
- [ ] Archive documentation on `mediawiki.org` (https://www.mediawiki.org/wiki/Extension:ClientSide)
- [ ] Remove from `mediawiki/extensions` / `mediawiki/skins` ( along with in `.gitmodules`)
- [ ] Remove tests in `integration/config` ( `zuul/layout.yaml` )
- [ ] Remove from `translatewiki` ( `groups/MediaWiki/mediawiki-extensions.txt` / `groups/MediaWiki/mediawiki-skins.txt` )
- [ ] Empty repository and add a OBSOLETE file.
- [ ] Archive the extension's diffusion repository: {rECLS}
- [ ] Mark the repository read-only in Gerrit https://gerrit.wikimedia.org/r/#/admin/projects/mediawiki/extensions/ClientSide
- [ ] Delete the GitHub mirror: https://github.com/wikimedia/mediawiki-extensions-ClientSide