Hello,
here the Microsoft's answer:
The -MFA handle was removed on purpose as it is no longer needed, authentication is done using the MS Assessment App cert and key,
MFA is fully supported while creating the app, there is no need to specify any cloud account when creating the task, you can simply run the Add-AzureAssessmentTask cmdlet without the parameter and it skips the cloud user step (the app info from the registry is auto detected when OMSAssessment runs)
Bye,
Luca