Well, think about it like this. React had several major versions in recent years, and did remove several APIs. Also the whole thing with the new hotness of the hooks API (which, honestly, I just truly don't get).
Angular had the whole thing with Angular 2, where they just shipped a whole new framework with the same name. So everyone with angular 1 codebase was, well, stuck with angular 1.
Don't know much about vue, but I think there was some drama about rather large API changes from Vue 2 to Vue 3. Correct me, if I'm wrong.
Backbone and Knockout, on the other hand, look fairly complete. Almost no chance of backwards incompatible changes there. So you can just write your code and be done with it.
But, honestly, one should use things that one knows. My current project is written in React. I'm fine with it. And if I had to start a greenfield one, I wouldn't use anything else, because I am proficient with React. I can deliver features. After all, our customers want the features, not the hot new framework.
Angular had the whole thing with Angular 2, where they just shipped a whole new framework with the same name. So everyone with angular 1 codebase was, well, stuck with angular 1.
Don't know much about vue, but I think there was some drama about rather large API changes from Vue 2 to Vue 3. Correct me, if I'm wrong.
Backbone and Knockout, on the other hand, look fairly complete. Almost no chance of backwards incompatible changes there. So you can just write your code and be done with it.
But, honestly, one should use things that one knows. My current project is written in React. I'm fine with it. And if I had to start a greenfield one, I wouldn't use anything else, because I am proficient with React. I can deliver features. After all, our customers want the features, not the hot new framework.