**Reason**
Per Anomie on https://gerrit.wikimedia.org/r/c/mediawiki/core/+/489894
> Personally I'd […] possibly see about archiving the extension in Gerrit.
> * T135291 has been open since 2016, with no activity other than hashar disabling CI tests.
> * https://github.com/webplatform/mediawiki-fxa-sso doesn't show any commits since 2014.
> * Nothing in Gerrit since 2014 besides global maintenance churn and a few cross-repo fixes like I29ca8847.
> * http://docs.webplatform.org/wiki/WPD:Projects/SSO/MediaWikiExtension has an expired security certificate (and redirects to a 404 if you bypass that).
> * http://docs.webplatform.org/ redirects to https://webplatform.github.io/docs/ which says the project was discontinued in 2015.
**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).
-- [x] Add #mediawiki-extensions-other if there is no project.
- Configuration/tests/integrations/etc.
-- [x] Remove from `mediawiki/extensions` / `mediawiki/skins` (along with in `.gitmodules`).
-- [x] Remove tests in [[ https://gerrit.wikimedia.org/g/integration/config | `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
-- [x] Empty repository and add an ARCHIVED file.
-- [ ] Archive the diffusion repository for the extension/skin: rEWPA
-- [x] Mark the repository read-only in Gerrit: https://gerrit.wikimedia.org/r/#/admin/projects/mediawiki/extensions/WebPlatformAuth
-- [ ] Delete the GitHub mirror: https://github.com/wikimedia/mediawiki-extensions-WebPlatformAuth