It’s a law aimed at a specific person. It fits the constitutional definition of a bill of attainder.
CC
I think a bill of attainder declares a particular person guilty of a crime—a common Parliamentary tactic of recent memory for the Founding Fathers. Of course the law is aimed at President Trump, but his name is nowhere in it. It would apply to anyone. In any case, as near as I can tell, the state of California is under no obligation by the Constitution to place anyone’s name on its Presidential ballot. I say, let them go for it. They think they’re being oh, so clever, but it will come back to bite them. In fact let all of the states controlled by the dems do it. I believe it will increase President Trump’s votes in the rest of the country.
On the other hand, excluding Lincoln from the ballot in 1860 did not end well for the democrat-controlled slave states. Perhaps that should give the democrats pause in 2020.