There is a known simple mitigation. Don't JIT random JavaScript, back to interpretation. Of course that means there is no use for V8, which is why it's not in this paper.
V8 has a JIT-free mode now, and it's still pretty damn fast in real world situations. It looks like in synthetic benchmarks they saw up to 80% decrease in performance, but in real-world applications they saw as little as a 6% decrease.
There is a known simple mitigation. Don't JIT random JavaScript, back to interpretation. Of course that means there is no use for V8, which is why it's not in this paper.