Cisco announced that Webex App will support Poly Headsets coming with the November 2021 update. Unfortunately still one Poly Headset (Poly Voyager Focus UC with BT600) is supported.
Poly support assured me that older models (e.g. Savi 400 series) will also be supported from Poly side, but it depends on Cisco integration.
Since many customers use these headsets, it would be very helpful if they were supported with the Webex app as soon as possible.