**Reason**:
This extension has been unmaintained since 2009, and has not worked since MediaWiki 1.23.
Note that there is an [[ http://wiki.4intra.net/CustomSidebar | unrelated CustomSidebar extension ]] used on a handful of wikis with a version number of `1.12+`.
- [ ] Archive documentation on `mediawiki.org` (https://www.mediawiki.org/wiki/Extension:CustomSidebar).
- [ ] Remove from `mediawiki/extensions` / `mediawiki/skins` (along with in `.gitmodules`).
- [x] Remove tests in `integration/config` (`zuul/layout.yaml`).
- [ ] Remove from `translatewiki` (`groups/MediaWiki/mediawiki-extensions.txt` / `groups/MediaWiki/mediawiki-skins.txt`).
- [ ] Remove manifest/other references from `mediawiki/vagrant` if they exist.
- [x] Empty repository and add an ARCHIVED file.
- [ ] ~~Archive Phabricator project `<#ProjectX>` for associated extension/skin.~~
- [ ] ~~Edit Phabricator project `<#ProjectX>` description for associated extension/skin with a link to this ticket.~~
- [ ] Archive the extension's diffusion repository: https://phabricator.wikimedia.org/diffusion/ECSB/.
- [x] Mark the repository read-only in Gerrit: https://gerrit.wikimedia.org/r/#/admin/projects/mediawiki/extensions/CustomSidebar.
- [ ] Delete the GitHub mirror: https://github.com/wikimedia/mediawiki-extensions-CustomSidebar.