**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+`.
**To-do list**
- This task
-- [ ] Add maintainers of that extension as subscribers to this task (if they have no apparent Phabricator account, notify them on-wiki or via email if possible and note it here).
-- [x] Add Phabricator project as a tag for this task, or #mediawiki-extensions-other if there is no project.
- On-wiki documentation
-- [x] Archive documentation on `mediawiki.org` (https://www.mediawiki.org/wiki/Extension:CustomSidebar): replace page contents with `{{Archived extension|last revision id before archiving|reason=archive reasons, per [[phab:T199712]]}}`.
-- [x] If documentation page was translatable, remove `<translate>`, visit Special:PageTranslation, and click "remove from translation" (if you don't have the translation administrator right, ask a [[ https://www.mediawiki.org/wiki/Special:ListUsers/translationadmin | user who does ]]).
-- [x] Update Wikidata item associated with documentation page
--- [x] add statement `Abandonware (Q281039)` to `instance of (P31)` together with qualifier `start time (P580)` = the YYYY-MM-DD (date that you decided to archive extension generally per edit history)
--- [x] add qualifier `end time (P582)` = the YYYY-MM-DD (same date as above) to `instance of (P31) = MediaWiki extension (Q6805426)`
- Phabricator
-- [x] ~~Archive Phabricator project for the extension.~~
-- [x] Mark all Phabricator tasks for the extension either Declined or Invalid.
-- [x] ~~Edit Phabricator project description for the extension with a link to this ticket.~~
- Translatewiki.net/translations
-- [x] Remove from `translatewiki.net` (`groups/MediaWiki/mediawiki-extensions.txt`) (add #translatewiki.net project as a tag for this task).
- Configuration/tests/integrations/etc.
-- [x] Remove from `mediawiki/extensions` (along with in `.gitmodules`).
-- [x] Remove tests in [[ https://gerrit.wikimedia.org/g/integration/config | `integration/config` ]] (`zuul/layout.yaml`).
-- [x] ~~Remove manifest/other references from `mediawiki/vagrant` if they exist.~~
-- [x] ~~Remove from https://doc.wikimedia.org/cover-extensions/ if present (requires #continuous-integration-infrastructure shell user to delete directly from the server).~~
- Repositories
-- [x] Empty repository and add an ARCHIVED file.
-- [x] Empty repository leaving only the README indicating the repo is archived and referring to this task by URL.
-- [x] Mark the repository read-only in Gerrit (Edit description to prepend "[ARCHIVED]" and append Task reference): (https://gerrit.wikimedia.org/r/#/admin/projects/mediawiki/extensions/CustomSidebar).
-- [ ] Archive the diffusion repository for the extension: (https://phabricator.wikimedia.org/diffusion/ECSB/).
-- [ ] Delete the GitHub mirror: (https://github.com/wikimedia/mediawiki-extensions-CustomSidebar).