I suspect that there are two groups of people who read the title of this blog post: those who are saying "oh finally!" and those who are unfamiliar with the term 'multi-factor authentication'.
As you are likely aware, a strong password is necessary but inefficient in today's world to protect your account from malicious attackers. The most common family of approaches to defend your account is referred to as "multi-factor authentication", which is exactly what it sounds like: relying on additional pieces of evidence beyond just your username and password in order to authenticate that you are who you say you are. (Have you ever had to type in a six-digit code that was texted to you, or click a link sent to your email? That's multi-factor authentication, too!)
Buttondown now lets you protect your account with the help of a third-party authenticator app like Google Authenticator or Authy. These apps exist as a remote fingerprint with a rotating code; when you log into Buttondown, you'll be prompted for the code from one of these apps.
If pictures are worth a thousand words, screenshares are worth at least fifteen hundred. Here's a two-minute walkthrough of what registering a QR code looks like and how to log in after doing so:
The support I've added for authenticator apps is one of many reasonable approaches that folks like to take when it comes to multi-factor authentication. I've already received requests for other avenues, such as Yubikey support and SMS support. I plan on expanding coverage in the future; if you have a particular use case you'd like to have me know about, please email me!