I used to be able to use the fob sensor to run "Arrive" scene when coming home

Yes, it is for now. You can use both but there are important differences. And remember that they have told us that the classic V2 app will be going away, they just haven’t said when.

Also, note that both app versions have a smart home monitor feature, but is completely different code. Dismissing an alert from one will not dismiss it any other app and so on.

See the following: