When you send the result of Object.freeze() to or from a WebWorker the object is still copied and comes out the other side as fully mutable (aka, not frozen).

Immutable JS is orthogonal in this case. That's "just" a fancy wrapper on top of a bunch of regular, mutable JS objects that makes it look immutable. It's not truly immutable to the runtime, and has no special interaction with webworkers as a result. It gets deep copied just like any other object.