**Reason**
According to https://wiki.openstreetmap.org/wiki/Slippy_Map_MediaWiki_Extension it's replaced by MultiMaps. Also it doesn't support ResourceLoader, still pretends its in SVN (see $wgExtensionCredits), etc. There's not even a mediawiki.org page that I could find for it.
**To-do list**
- This task
-- [x] 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 Phabricator project <#ProjectX> as a tag for this task, and remove #mediawiki-extensions-other/#other-skins as appropriate.
- On-wiki documentation
-- [x] ~~Archive documentation on `mediawiki.org` (provide URL): replace page contents with `{{Archived extension|last revision id before archiving|task=T######}}` (for extensions)/`{{Archived skin|last revision|task=T######}}` (for skins); replace `T######` with this task's number.~~
-- [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 (provide URL) 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/skin (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)/MediaWiki skin (Q21996535)`~~
- Phabricator
-- [x] Mark all Phabricator tasks for the extension/skin either Declined or Invalid. Add a comment pointing to this task when doing so for reference.
-- [x] ~~Archive Phabricator project <#ProjectX> for the extension/skin.~~
-- [x] ~~Edit Phabricator project <#ProjectX> description for the extension/skin 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 ]] / [[ https://gerrit.wikimedia.org/r/plugins/gitiles/translatewiki/+/refs/heads/master/groups/MediaWiki/mediawiki-skins.txt | groups/MediaWiki/mediawiki-skins.txt ]]).
- Configuration/tests/integrations/etc.
-- [x] 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 ]]).
-- [x] ~~Remove manifest/other references from [[ https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/vagrant/+/master | mediawiki/vagrant ]] if they exist.~~
-- [x] ~~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, leaving only the README indicating the repo is archived, and referring to this task by URL.
-- [x] Remove from [[ https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/extensions/+/master | mediawiki/extensions ]] (along with in `.gitmodules`).
-- [x] Mark the repository read-only in Gerrit (also edit description to prepend "[ARCHIVED]" and a reference to this task): (https://gerrit.wikimedia.org/r/admin/repos/mediawiki/extensions/OpenStreetMapSlippyMap).
-- [ ] Archive the Diffusion repository for the extension/skin: ({rEOSM}).
-- [x] Delete the GitHub mirror: (https://github.com/wikimedia/mediawiki-extensions-OpenStreetMapSlippyMap).