IANAL either, but I don't see why it would be? Just have a separate cluster type, e.g SLA Zonal, SLA Regional. The SLA already differentiates the current cluster types. Anthos Clusters are also not subject to any additional fees?
And having it opt-in will save face with those users of GKE where an additional $73/m is significant.
And having it opt-in will save face with those users of GKE where an additional $73/m is significant.