**Reason**:
It is not deployed anywhere according to wikiapiary
Barely any code has been since 2012 beside the routine maintenances.
- [x] Archive documentation on `mediawiki.org` https://www.mediawiki.org/wiki/Extension:Semantic_Expressiveness
- [x] Remove from `mediawiki/extensions` ( along with in `.gitmodules`)
- [x] Remove tests in `integration/config` ( `zuul/layout.yaml` )
- [x] Remove from `translatewiki` ( `groups/MediaWiki/mediawiki-extensions.txt` )
- [x] Empty repository and add a OBSOLETE file.
- [x] Archive the extension's diffusion repository: rESEX
- [x] Mark the repository read-only in Gerrit: https://gerrit.wikimedia.org/r/#/admin/projects/mediawiki/extensions/SemanticExpressiveness
- [ ] Delete the GitHub mirror: https://github.com/wikimedia/mediawiki-extensions-SemanticExpressiveness