Sadly, Clef is shutting down in a couple months. If you haven't heard of it, it was a clever way to use your email and a mobile app to sign in to websites. I had integrated Clef logins to indieauth.com as one way to authenticate your email address. Since they are shutting down in June, I am proactively removing it from the website right now.
The other method for authenticating your email address is is to receive a verification code sent to your address, which I will of course continue to support.
While I was in the code, I figured it was about time to retire SMS authentication as well. While conceptually receiving a code via SMS or email are equivalent, in practice this is not true. SMS is actually a pretty insecure protocol, and it's possible (and not that expensive) to intercept SMS messages to all phones in a nearby area if you have the right hardware.
So as of today, SMS authentication is no longer an option for indieauth.com, and you will have to receive verification codes sent to your email instead of using the Clef app. I may bring back SMS support in indieauth.com as a second factor authentication in the future.
Here is my list of providers I use now.