X now supports passkeys on iOS in the US

When you set up passkeys for an account, your device generates one public key and one private key. The private key stays on your device, while the shared public key is stored on the platform you want to sign into (in this case, X). Once you’re all set up, you can choose a passkey option instead of a password to log in to an X account. Your device will authenticate your identity using the public key. The same passkey will work across all devices that are signed into the same iCloud account.