The latest news on this front comes from a really interesting article at Ballot Access News written by the widely-respected election expert and advocate for third party/independent ballot access Richard Winger.
On August 6, the California Secretary of State filed this brief in the federal lawsuits over the new California law on presidential tax returns. Although there are several related cases, it seems simpler to refer to this case as Trump v Padilla, e.d., 2:19cv-1501. The brief relates to all the federal cases.
The state says that requiring presidential candidates to reveal their tax returns is not a qualification because any candidate is free to reveal the returns.
Previous cases have held that requiring federal candidates to take an oath that they do not belong to a group that advocates the violent overthrow of the government is unconstitutional. That type of oath has been held to be a qualification.