Adding the PR here as well:
https://github.com/discourse/discourse/pull/39000
Settings will be migrated if ai_translation_enabled or ai_translation_backfill_limit_to_public_content have been modified to non-default values. Untouched otherwise.
ai_translation_backfill_limit_to_public_content = true will now be
ai_translation_target_categories=<all public categories>ai_translation_personal_messages=none
Conversely, if ai_translation_backfill_limit_to_public_content = false, it will now be
ai_translation_target_categories=<all categories>ai_translation_personal_messages=all
ai_translation_personal_messages also allows a group setting.