This is exactly how it currently works:

  $ fly volumes create mydata
  Warning! Individual volumes are pinned to individual hosts.
  You should create two or more volumes per application.
  You will have downtime if you only create one.
  Learn more at https://fly.io/docs/reference/volumes/
  ? Do you still want to use the volumes feature? (y/N)

(and yes, the warning is already even in red letters too)