The constitution prohibits “ex post facto” laws - how can you revoke someone’s documentation when they complied with the laws as they were at the time?

I agree with you generally that this should be illegal, but it probably isn't.

It definitely isn't ex post facto; this is not a law that punishes anyone from a legal perspective, it merely changes the requirements for a certain privilege (the ability to drive a vehicle). If it declared these licenses invalid before the date of the law (which could carry punishments for illegally operating vehicles), then it would be ex post facto.

Another way to put it is that it simply makes a certain action illegal which was previously legal, and laws do that sort of thing all the time. Consider that in the US you didn't need a driver's license in order to drive at all until 1913. The NJ law requiring drivers licenses also "revoked" someone's privilege even though they complied with the laws previously, requiring them to get a permit from then on. But, since it didn't introduce any punishments for not having the permit before it was introduced, it wasn't ex post facto.

Of course the law is also clearly discriminatory, but US's extremely limited anti-discrimination laws are likely not broad enough to be applied here.

The current events should awaken many people to the sad fact that US laws and its entire legal system exists primarily to protect the wealthy and the powerful from everyone else; all other functions are secondary. As such, many horrible, immoral, and unjust things are legal under US laws, and many others will be twisted into being legal by the supreme court.