@Administration Apologies for the delayed response on this post.
Even though you disable registration campaign, end users can still use SMS/Voice/Emails as a form of MFA.
Refer to this QnA - https://learn.microsoft.com/en-us/answers/questions/1307030/changes-to-the-registration-campaign-feature-in-az where one of our Program manager @Luc van den Ende confirmed the same in this post.
Let me know if you have any further questions, feel free to post back.
Please remember to "Accept Answer" if answer helped, so that others in the community facing similar issues can easily find the solution.