If you want to enable two-factor authentication for Nextcloud on a per-user basis, it's just a simple app installation away.

At krypt.co we’re building the next generation of mobile-based authentication. Our vision is that authentication should be based on strong cryptography.

List of websites and whether or not they support One Time Passwords (OTP) or Universal 2nd Factor (U2F).
Also see the list of dongles and the protocol they support.

List of websites and whether or not they support 2FA.