My assumption would be that it would drastically reduce the attack to be borderline irrelevant. I've never turned on Under Attack so somebody else may have more insight and the docs[1] don't describe precisely what happens besides a JS interstitial.

I know that JS challenges, both interactive and non-interactive, can be solved by bots. I've seen it. However, I suspect that the challenges just get harder and harder until the attack levels drop.

[1] https://developers.cloudflare.com/fundamentals/reference/und...