Are you thinking of affirmative action?

Affirmative action is by design discriminatory, but not against nationality. It's discriminatory based on race and sex. So I think your grudge is not striking the right target. And in any case, affirmative action has been mostly wound down, which began to happen when Obama was President. Not because he did anything, but because SCOTUS declared that his election was evidence that affirmative action was no longer required and thus ruled against it in new cases.

Any source? In my field US Citizens and permanent residents are actually preferred for at least two reasons, first they are eligible for graduate grants like NSF so they are not using department's money; second upon graduation they are eligible for more jobs because places like national labs do not hire foreigners.

I don't think I experienced discrimination during admissions either. Off the top of my head, I don't know any US citizens who told me that they wanted to go to grad school but were unable to be admitted to a school.