Reason
Apparently broken since 1.35 per posts on its talk page

To-do list

• This task
  • Add maintainers of that extension/skin 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).
  • Add Phabricator project <#ProjectX> as a tag for this task, and remove MediaWiki-extensions-Other/Other-skins as appropriate.
• On-wiki documentation
  • Archive documentation on mediawiki.org (https://www.mediawiki.org/wiki/Extension:SideBarMenu): replace page contents with {{Archived extension|last revision id before archiving|task=T347216}}
  • 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 user who does).
  • Update Wikidata item (https://www.wikidata.org/wiki/Q21678565) associated with documentation page
    • 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/skin (generally per edit history)
    • add qualifier end time (P582) = the YYYY-MM-DD (same date as above) to instance of (P31) = MediaWiki extension (Q6805426)/MediaWiki skin (Q21996535)
• Phabricator
  • Mark all Phabricator tasks for the extension either Declined or Invalid. Add a comment pointing to this task when doing so for reference.
  • Archive Phabricator project <#ProjectX> for the extension/skin.
  • Edit Phabricator project <#ProjectX> description for the extension with a link to this ticket.
• Translatewiki.net/translations
  • Remove from translatewiki.net (groups/MediaWiki/mediawiki-extensions.txt)
• Configuration/tests/integrations/etc.
  • Remove tests in integration/config (zuul/layout.yaml).
  • Remove manifest/other references from mediawiki/vagrant if they exist.
  • Remove from https://doc.wikimedia.org/cover-extensions/ if present (requires Continuous-Integration-Infrastructure shell user to delete directly from the server).
• Repositories
  • Empty repository, leaving only the README indicating the repo is archived, and referring to this task by URL.
  • Remove from mediawiki/extensions / mediawiki/skins (along with in .gitmodules).
  • Mark the repository read-only in Gerrit (also edit description to prepend "[ARCHIVED]" and a reference to this task): (https://gerrit.wikimedia.org/g/mediawiki/extensions/SideBarMenu).
  • Deactivate the Diffusion repository for the extension/skin: (rESBM).
  • Delete the GitHub mirror: (https://github.com/wikimedia/mediawiki-extensions-SideBarMenu).