Reason
I'd like to suggest that MediaWiki-extensions-ParserMigration be archived:

• The documentation at https://www.mediawiki.org/wiki/Extension:ParserMigration says that this extension was for replacing tidy, which appears to be complete - the extension was deployed until 2018, but isn't anymore
• It has no recent changes: other than commits by libraryupgrader and Translation updater bot, the five most recent code changes are:
  • Split apihelp messages to a separate file by Krenair in August 2019
  • build: Upgrade phan-taint-check-plugin from 1.5.x to 2.0.1 by James Forrester in July 2019
  • Remove empty phan stubs directory by Legoktm in March 2019
  • Migrate to newer phan by Legoktm in February 2019
  • Don't manually suppress PhanDeprecatedProperty by Legoktm in June 2018
• As part of cleaning up the EditPage class, deprecations and breaking changes will need to be made; the extension's MigrationEditPage class extends EditPage and would likely be broken unless effort was dedicated to maintaining it

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 MediaWiki-extensions-ParserMigration as a tag for this task, and remove MediaWiki-extensions-Other as appropriate.
• On-wiki documentation
  • Archive documentation on mediawiki.org (here): replace page contents with {{Archived extension|3781121|reason=Only used during the migration from tidy; see {{phab|T250936}} for more details.}}
  • 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/Q25860665) 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 MediaWiki-extensions-ParserMigration
  • Edit Phabricator project MediaWiki-extensions-ParserMigration 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): Gerrit repository
  • Archive the Diffusion repository for the extension: rEPMI
  • Archive the GitHub mirror: https://github.com/wikimedia/mediawiki-extensions-ParserMigration