**Reason**
Per its own page, "Stopped working somewhere around 1.17."
**To-do list**
- This task
-- [X] Add maintainers of that extension 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). (I added Ryan Lane, who created the extension page. The listed maintainer, Mathias Lidal, has no Phabricator account I can find, and has been globally inactive since 2015)
-- [X] Add Phabricator project #mediawiki-extensions-smoothgallery as a tag for this task, and remove #mediawiki-extensions-other as appropriate.
- On-wiki documentation
-- [X] Archive documentation on `mediawiki.org` (https://www.mediawiki.org/wiki/Extension:SmoothGallery): replace page contents with `{{Archived extension|3837156|reason=Hasn't worked since around 1.17. Also see {{phab|T252273}}.}}` (for extensions)
-- [X] If documentation page was translatable, remove `<translate>`, visit Special:PageTranslation, and click "remove from translation" (if you don't have the translation administrator right, ask a [[ https://www.mediawiki.org/wiki/Special:ListUsers/translationadmin | user who does ]]).
-- [X] Update Wikidata item (Q21678606) associated with documentation page
--- [X] add statement `Abandonware (Q281039)` to `instance of (P31)` together with qualifier `start time (P580)` = the YYYY-MM-DD date that you decided to archive extension (generally per edit history)
--- [X] add qualifier `end time (P582)` = the YYYY-MM-DD (same date as above) to `instance of (P31) = MediaWiki extension (Q6805426)`
- Phabricator
-- [X] Mark all Phabricator tasks for the extension either Declined or Invalid. Add a comment pointing to this task when doing so for reference.
-- [X] Archive Phabricator project #mediawiki-extensions-smoothgallery for the extension.
-- [X] Edit Phabricator project #mediawiki-extensions-smoothgallery description for the extension with a link to this ticket.
- Translatewiki.net/translations
-- [ ] Remove from `translatewiki.net` ([[ https://gerrit.wikimedia.org/r/plugins/gitiles/translatewiki/+/refs/heads/master/groups/MediaWiki/mediawiki-extensions.txt | groups/MediaWiki/mediawiki-extensions.txt ]]
- 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 ]]).
-- [ ] Remove manifest/other references from [[ https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/vagrant/+/master | 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, leaving only the README indicating the repo is archived, and referring to this task by URL.
-- [ ] Remove from [[ https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/extensions/+/master | mediawiki/extensions ]]
-- [ ] Mark the repository read-only in Gerrit (also edit description to prepend "[ARCHIVED]" and a reference to this task): https://gerrit.wikimedia.org/g/mediawiki/extensions/SmoothGallery.
-- [ ] Archive the Diffusion repository for the extension: rESMG
-- [ ] Archive the GitHub mirror: https://github.com/wikimedia/mediawiki-extensions-SmoothGallery