"you can't be racist against white people"
The left's messaging on this in the last twenty years has been a mess. This phrasing being a key example.
What they pushed is: "You can be
prejudiced against anybody, but you can only be
racist against marginalized minorities" because "racism equals prejudice plus power." This is incredibly complicated, this is adding steps that don't need to be there. What they
should have pushed is: "You can be
racist against someone of any ethnicity, but is rare for racism against whites to happen at the
institutional level. Usually marginalized minorities like black people are pushing back against
institutionalized racism."
The former narrative has everybody competing to see who's more marginalized ergo who experiences the most "prejudice plus power" ergo who deserves the most accommodations to counter it. It has white people arguing that they're institutionally discriminated against so that they don't lose out to things like affirmative action and DEI. If they'd gone with the latter narrative it would have helped people properly contextualize the struggles that marginalized minorities face.
Most people hear "You can't be racist against white people" and think, "Well that's bullshit," because it is. If they'd heard "You can be racist against white people, sure, but black people generally experience
more and
different racism," they would also have conceded the point. I think this is a massive marketing failure on the left's part and has helped get us where we are in terms of racial division in the U.S and Canada.
"Race is a social construct"
This, on the other hand, should just be apparent to people. "White" is the best example of this, it's an ever-moving goalpost. Over a hundred years "white" has been a half-dozen different things in America. The only reason people ever think "white" is real is because the people in power that dictate what "white" is have let them into the fold. Seeing Irish-American white supremacists is so funny to me.