**Reason**: https://gerrit.wikimedia.org/r/#/admin/projects/mediawiki/extensions/Phalanx is marked as read-only, and the extension page on MediaWiki is marked as unmantained and T70974 indicates that indeed the extension is abandoned.
- [x] Archive documentation on `mediawiki.org` https://www.mediawiki.org/wiki/Extension:Phalanx
- [x] Empty repo and note archive status
- [x] Remove from `mediawiki/extensions` (along with in `.gitmodules`) [was not there]
- [x] Remove tests in `integration/config` (`zuul/layout.yaml`) [already done]
- [x] Remove from `translatewiki` (`groups/MediaWiki/mediawiki-extensions.txt` )
- [ ] Archive the extension's diffusion repository: rEPHA
- [ ] Delete the GitHub mirror: https://github.com/wikimedia/mediawiki-extensions-Phalanx (needs someone in #acl*releng)