The Maintenance::runChild method was marked as stable to override until it was renamed to Maintenance::createChild, leaving the old name as a deprecated alias. Because it was marked as stable to override we might need to wait for a release cycle to ensure that anyone who is overriding it can rename their overrides.
This task is to replace usages of Maintenance::runChild with ::createChild once this is appropriate per the stable interface policy and to do the pre-work necessary to reach this point.