When will passkey support be introduced to 1Password on iOS?

However once I entered my username / password combo I could sign in via passkey quite easily. Since 1password autofills it, it isn't so much of a problem, it's still very quick indeed.

You need to upgrade your passkey to support login, and not just 2FA. You first need to enable Passkey logins in experimental features:

Then at https://github.com/settings/security you should be able to upgrade your passkey (or register a new one).