Can you clarify what you are referring to with 2FA for your account being restricted to Plus accounts? All merchant and staff accounts have the ability to enable 2FA security for account login. You can review the steps and more information on enabling this in our Help Center here: Shopify Help Center | Two-step authentication.
For customer accounts, you are correct that we don't have 2FA as an option at this time. New customer accounts, which also support b2b, are not password dependent and are activated through the customer email. All customer accounts require that verification step by receiving an email and activating the account through the link sent to them.
Recaptcha is available for things like blog comments, where an account is not directly required.
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