In Ontario, access to these programs was gated by an IQ test given to all students based on the outcome of a standardized test (this was ~30 years ago, no idea what they do today). I'm not saying it was perfectly objective or equitable but it was a start at trying to make it objective. Are programs not doing something similar in California or elsewhere in the US?

Yep - I remember the CCAT from 4th grade that resulted in my being placed into a different class for 5th. AFAIK, we were given this test "cold" (no prep) and I remember it being timed.

It’s literally illegal to give students IQ tests in California.