Hi @joffrey_sb
Welcome back to the Auth0 Community!
I do believe that the error that you’re seeing is related to the new Unified Phone Experience feature, which will be the recommended solution for phone providers moving forward and Legacy Passwordless would refer to the “old” specific provider.
From what information I was able to gather, there is no end date to this deprecation quite yet as we’re giving customers time to try out the new experience, switch back and forth as needed and test it out. This should not affect your service or cause any issues, eventhough seeing a warning message can understandably raise alarm bells ( thank you for your due diligence! ).
I do encourage you taking a look through our documentations about the new feature, which I linked above.
Hope this helped!
Gerald