I've noticed several websites that exclusively a single-use login code for authentication. This is a 4-digit or 6-digit login code sent via email instead of a password, it's not 2FA or a passkey. (I would rather they don't do this as it means I have to open my email instead of using a password ...
Source: [Hacker News](https://news.ycombinator.com/item?id=49560168)