They say passkey will be a the coming replacement of all passwords and authentication systems. But on Google Tumbleweed is not listed as eligible OS. Why is it so?
This page does not mention any Linux distribution at all. I am not sure why you pick on Tumbleweed specifically. And is not Google a better place to ask about why they do or do not do something?
At first I didn’t want to mention any distribution name, but as my computer runs with Tumbleweed maybe it’s an issue with that.
As you mention Google - who am I? It would be the business of a big distribution like openSUSE to make Google mention Linux. Quite often only Windows and Android are listed. If some publicist from openSUSE who will coordinate with RHEL, Debian, Manjaro and/or Ubuntu will lead Google to include Linux into the passkey initiative. Certainly it will take some time.
Nothing on this page is really Ubuntu specific. Everything is about using Passkeys in browsers and storing them on the standard USB security devices. You have the same browsers and the same KeePassXC in Tunbleweed and can plug in the same USB security keys. Try it and tell others your experience.