My OS (Arch), my browser (Firefox) and my hardware (Yubikey) all work fine with some sites. For example my Google account credentials are on my Yubikey passkey. But half of the sites that claim to support passkeys won't offer me the option to save the passkey on my hardware. Paypal is one such example. Facebook is another. I don't understand why. They seemingly only want to cooperate with cloud based passkey storage. I don't know what mechanics govern this.

Paypal refuses to offer generating a passkey on either Firefox or Chrome on Linux. Spoofing the UA to Firefox on Windows does work, though. Whatever they're doing, it's not normal. I'm surprise you run into it so often.

This is the first time I've seen this issue, very weird. Then again, I had to apply some kind of hack to enable TOTP back in the day (something about faking an RSA key fob), so I'm not surprised Paypal is stil shit.