Once migration to OOUI is complete, as a follow up to T111088
Change 366872 had a related patch set uploaded (by Jforrester; owner: Jforrester):
[mediawiki/core@master] [WIP] EditPage: Remove legacy non-OOUI render mode
Change 366872 merged by jenkins-bot:
[mediawiki/core@master] EditPage: Remove legacy non-OOUI render mode
Done. Some follow-up to get rid of the old CSS class name in https://gerrit.wikimedia.org/r/373164 and to get rid of the EditPage::isOouiEnabled() method which is called by Translate and TemplateSandbox.