it is allowed but not simple to integrate nor does paypal offer help for this, you need to ask yubikey. usage, in german, you need to translate yourself. https://www.paypal-community.com/t5...t-mehr/m-p/2736986?profile.language=de#M41557 in english https://www.reddit.com/r/yubikey/comments/ngxmep/yubikey_for_2fa_on_paypal/
Can you still only add one YubiKey to PayPal (not multiple)? (Pretty dangerous in case you lose the key or when it stops functioning)