React is also pretty much finished. I have not experienced breakage except in some exotic functionality. The churn comes from switching to hooks, but that can happen gradually and is not strictly necessary. Oh, and of course library authors like to break stuff, but that is not on React itself.
Before the switch to hooks there was the switch from class components to functions, but some features (error capture) were only available using class component.
The router situation has always been a mess too, and passing props around and lately contexts also overlap and have changed with time.
Some of building blocks seem to have been built organically, without regards for backwards compatibility. It is still very useful (I happily use react and next js) but the concepts are not very elegant and internally consistent IMHO.
