I remember sites doing all sorts of hacks to identify and shut down IE back in the day. "Works best in Chrome/Firefox".
"The proposal calls for at least the following information in the signed attestation:
The attester's identity, for example, "Google Play".
A verdict saying whether the attester considers the device trustworthy.
"So a user agent string and a weak attestation?
This seems an overcomplex nothingburger.
That's not a "nothingburger" IMHO.