That would be highly illegal: it'd be discrimination on the basis of race (which is protected under the law) rather than on the basis of immigration status (which is not protected).

For it to be illegal you have to prove intent.

The incentives ensure that it will happen with zero intent, and probably without the people doing it even realize they're doing it. It's not illegal to see someone, think of them as a 'sucker' but not even realize why, then lowball them, which is far more likely than for a person to actually consciously confront themselves they may be a racist.

In any case, even if they know it's illegal, it's not so easy to enforce, the fact that people get successfully sued or jailed a small fraction of the time isn't going to be some solace.

The only way to actually solve it is to remove the incentive in place, namely either the market pressure to get the best developer at the cheapest price or the vulnerability of being an immigrant.

We've all seen hiring managers that coincidentally hired only or nearly only their fellow countrymen, and nothing happens to them, even though it is highly illegal.