Reason:
Unmaintained, with a documented XSS vulnerability since April 2017; WikiApiary only lists two wikis using it, one on MediaWiki 1.27 and the other on 1.25, and the extension page itself claims the extension doesn't work in 1.29.1.

(As a procedural note, could someone who has permission to configure forms, sync the Cleanup form with the checklist template and add MediaWiki-extensions-Other and Other-skins as default tags?)

To-do list

• This task
  • Add maintainers of that extension as subscribers to this task (@Subfader; I was unable to find a Phabricator account associated with Roman Allenstein).
  • Add Phabricator project <#ProjectX> as a tag for this task, or MediaWiki-extensions-Other/Other-skins if there is no project.
• On-wiki documentation
  • Archive documentation on mediawiki.org (https://www.mediawiki.org/wiki/Extension:ExternalLinks).
  • Archive Wikidata item associated with mediawiki.org documentation page (https://www.wikidata.org/wiki/Special:EntityPage/Q21676597) (remove nearly all statements, except instance of (P31) = MediaWiki extension (Q6805426), which should add a qualifier end time (P582) = the YYYY-MM-DD date that you decided to archive extension, generally per edit history, change English (and other languages if possible) description to "archived MediaWiki extension" (and its translation)).
• Phabricator
  • Archive Phabricator project <#ProjectX> for associated extension/skin.
  • Mark all Phabricator tasks for the extension/skin either Declined or Invalid.
  • Edit Phabricator project <#ProjectX> description for associated extension/skin with a link to this ticket.
• Translatewiki.net/translations
  • Remove from translatewiki.net (groups/MediaWiki/mediawiki-extensions.txt / groups/MediaWiki/mediawiki-skins.txt).
• Configuration/tests/integrations/etc.
  • Remove from mediawiki/extensions / mediawiki/skins (along with in .gitmodules).
  • 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 and add an ARCHIVED file.
  • Archive the extension's diffusion repository: (provide the Diffusion repository callsign or URL).
  • Mark the repository read-only in Gerrit: (provide Gerrit Repository URL).
  • Delete the GitHub mirror: (provide GitHub repository URL).