iPad Pro 12.9-inch (2020) is now compatible, with the use of the Osmo Reflector for iPad (2021).
The Osmo Reflector for iPad (2021) makes it possible to use Osmo with all thin bezel iPads (iPad Air 4, iPad Pro 11-inch, iPad Pro 12.9-inch).
Note that iPad Pro 12.9-inch (2021) is not compatible yet, but is expected to be made compatible by the end of October 2021.