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

@Puri_sm is happy to announce the addition of the Librem Key to the product line. Be able to fully secure your laptop and quickly determine if your laptop has been compromised at the bios level!
source : https://twitter.com/Puri_sm/status/1042977718868951040

Solo.
The first, open source, security key supporting FIDO2.
An upgrade to U2F Zero.

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