Given the ubiquity of react, I think immutability is generally rated pretty appropriately. If anything, I think mutability is under-rated. I mean, it wouldn't be applicable to the domain of Temporal, but sometimes a mutable hash map is a simpler/more performant solution than any of the immutable alternatives.
replyProps data passed to React itself isn't immutable which is probably one of the missing bricks.
replyReact only checks references but since the objects aren't immutable they could have changed even without the reference changing.
Immutability also has a performance price which is not always great.
Yes, you can mutate props. But no, it's probably not going to do what you want if you did it intentionally. If react added Object.freeze() (or deepFreeze) to the component render invoker, everything would be the same, except props would be formally immutable, instead of being only expected to be immutable. But this seems like a distinction without much of a difference, because if you just try to use a pattern like that without having a pretty deep understanding of react internals, it's not going to do what you wanted anyway.
replyReact doesn’t really force you to make your props immutable data. Using mutable data with React is allowed and just as error prone as elsewhere. But certainly you are encouraged to use something like https://immutable-js.com together with React. At least that’s what I used before I discovered ClojureScript.
replyImmutability is often promoted to work around the complexity introduced by state management patterns in modern JS. If your state is isolated and you don't need features like time travel debugging, mutable data structures can be simpler and faster. Some so-called immutable libraries use hidden mutations or copy-on-write, which can actually make things slower or harder to reason about. Unless you have a specific need for immutability, starting with mutable structures is usually more sane.
replyWell, mutability is the default, and React tries to address some of the problems with mutability. So React being popular as a subecosystem inside a mutable environment isn't really evidence that people are missing out on the benefits of mutability.
replyThough React is less about immutability and more about uni-directional flow + the idiosyncrasy where you need values that are 'stable' across renders.