It's legal, just difficult to defend, because there are racial differences in IQ. So, there is a presumption by the EEOC that you are discriminating based on race if you give an IQ test.

You can get around this by conducting a study that finds IQ is better correlated to performance on a particular role than any other nondiscriminatory measure, but good luck with that, and good luck proving that when you get sued by someone. It's just too risky, despite not being illegal.

It's absolutely legal. Like any other screening tool that has significantly differential impact by race, it's only legal when it is sufficiently demonstrably related to performance in the specific jobs for which it is used as a screening tool.

There is a popular myth that it is illegal because of a case where it was found to be illegal in the specific manner in which it was used.

https://en.m.wikipedia.org/wiki/Griggs_v._Duke_Power_Co.#

Pretty sure that's not the case. When I worked for IBM back in the day, their interview did in fact involve an actual IQ test administered online as a first screening step.

As with all things employment law, it's more complicated than that. Briefly, the test must be "professionally developed" and relevant. Thus the whiteboard programming exam is an admissable proxy for an IQ test.

https://www.hiresuccess.com/blog/is-employment-testing-legal

I've been given an actual IQ test during an interview process in the U.S.