Reason
Extension seems to be nothing more than a way to bring in the solarium library, and as such, a copy paste of code

It's available to install via composer at https://packagist.org/packages/solarium/solarium

To-do list

• This task
  • 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).
  • Add Phabricator project <#ProjectX> as a tag for this task, and remove MediaWiki-extensions-Other as appropriate.
• On-wiki documentation
  • Archive documentation on mediawiki.org (https://www.mediawiki.org/wiki/Extension:Solarium): replace page contents with {{Archived extension|3214638|reason=Extension seems to be nothing more than a way to bring in the solarium library, and as such, a copy paste of code. See {{phab|T252911}}.}}
  • 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 user who does).
  • Update Wikidata item (https://www.wikidata.org/wiki/Q21678678) associated with documentation page
    • 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)
    • add qualifier end time (P582) = the YYYY-MM-DD (same date as above) to instance of (P31) = MediaWiki extension (Q6805426)
• Phabricator
  • Mark all Phabricator tasks for the extension either Declined or Invalid. Add a comment pointing to this task when doing so for reference.
  • Archive Phabricator project <#ProjectX> for the extension.
  • Edit Phabricator project <#ProjectX> description for the extension with a link to this ticket.
• Translatewiki.net/translations
  • Remove from translatewiki.net (groups/MediaWiki/mediawiki-extensions.txt).
• Configuration/tests/integrations/etc.
  • Remove tests in 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
  • Empty repository, leaving only the README indicating the repo is archived, and referring to this task by URL.
  • Remove from mediawiki/extensions (along with in .gitmodules).
  • 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/Solarium).
  • Archive the Diffusion repository for the extension: (rESLA extension-Solarium).
  • Delete the GitHub mirror: https://github.com/wikimedia/mediawiki-extensions-Solarium