The first commit of [Authlib] happened on Oct 21 2017, but it really started a very long time ago when Flask-OAuthlib was first introduced. The idea was to create a replacement for Flask-OAuthlib, since it was not well designed (or even implemented wrong).
I find it really hard to continue the development of Flask-OAuthlib, both on code and time. At that moment, OAuthlib, the library it depends was in a very bad status (now it is moved into an org), the APIs it provided are not good enough, or not of my taste. And I barely have much time or enthusiasm to fix the issues.
That's why I created [Authlib] with a sustainable idea from the beginning. Get a better idea by reading Announcement of Authlib.