Does the term have any meaning now, or has it become simply a slur for anyone whose political beliefs differ, like "communist" or "terrorist" before it?
In some ways, I'd argue we should then be even more vigilant because white supremacist rhetoric / apologist sentiment and actions are having an easier time to hide behind the veils of fuzziness.
One comment though: you do not have to be white to be a white supremacist. White supremacy is the belief that white people are superior to people of other races, and should thus dominate them.
We see this pattern in other arenas too, e.g. gays and homophobia or women and gender bias.
Look at the out-of-context quotes given in the first link and decide for yourself whether this is enough to justify a charge of being a white supremecist, even given that the article is explicitly dedicated to working towards that conclusion. Don't just take their word for it. I'll gladly concede that Miller is anti-immigration to the point of being xenophobic, but white supremecist is a huge stretch.
[1] https://www.splcenter.org/hatewatch/2019/11/12/stephen-mille...
[2] https://www.splcenter.org/stephen-miller-breitbart-emails
"For almost three decades, Jared Taylor has been publishing his ideas about race at the American Renaissance magazine and now at a website called AmRen, which is considered a mouthpiece for white supremacist ideology. ... The website is not well-known outside white nationalist circles — but it found an audience in White House adviser Stephen Miller."