Proximity is for use with on-prem registered endpoints, not those registered to the Webex Cloud.
We are migrating our on-premise devices to the cloud and proximity is not supported? This is a huge gulp. This is our primary source for connecting to the equipment. I don't understand what the limitation is? It's still the same hardware in the room. You'd think Cisco would want everyone to leverage the cloud. I don't think Proximity and Webex teams are comparable and I don't want another tool forced on the users.
I understand your concern. Cloud endpoints do Proximity ultrasound pairing, but use an different message format. Due to the nature of on prem vs cloud, and the local network requirements of Cisco Proximity, having on-prem Proximity and cloud endpoints communicate directly is quite possibly unfeasible due to security concerns.
That said, both Webex Teams and Meetings support on-prem and cloud endpoints. Maybe even Jabber, not sure about that one.