**Reason**
Per {T345274}, this service was decommissioned from Wikimedia production and is unowned and unused. Let's archive it.
Note that the #similarusers project is **also** used for the replacement work for it, https://gerrit.wikimedia.org/g/mediawiki/extensions/SimilarEditors, which is not archived.
**To-do list**
- This task
-- [x] Add maintainers of that service 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 <#similarusers> as a tag for this task
- On-wiki documentation
-- [ ] Archive https://wikitech.wikimedia.org/wiki/Similarusers
- Phabricator
-- [ ] Archive the Phabricator project tag and make the project description point to this very task.
-- [ ] Mark all relevant Phabricator tasks for the service Declined.
- Configuration/tests/integrations/etc.
-- [ ] Remove tests in [[ https://gerrit.wikimedia.org/g/integration/config | integration/config ]] ([[ https://gerrit.wikimedia.org/r/plugins/gitiles/integration/config/+/master/zuul/layout.yaml | zuul/layout.yaml ]]).
- Repositories
-- [ ] Empty repository, leaving only the README indicating the repo is archived, and referring to this task by URL.
-- [ ] Mark the repository read-only in Gerrit (also edit description to prepend "[ARCHIVED]" and a reference to this task, remove all access lists and reparent on `All-Archived-Project`): https://gerrit.wikimedia.org/r/admin/repos/mediawiki/services/similar-users,general
-- [ ] Deactivate the Diffusion repository for the service: https://phabricator.wikimedia.org/diffusion/MSSU/
-- [ ] Delete the GitHub mirror: https://github.com/wikimedia/mediawiki-services-similar-users