Possibly - not sure which cases have dealt specifically with national security letters and which have dealt with other aspects of the related programs. However

Possibly - not sure which cases have dealt specifically with national security letters and which have dealt with other aspects of the related programs. However there have definitely been cases which the government lost initially and then won on appeal, and as I've discussed on here before, I'm not convinced that inherently secret agencies will actually not do illegal things once they lose the case. They may still send letters to intimidate people and just won't be able to enforce it so easily. So the concept is still relevant IMO. We can't assume companies are allowed to disclose such backdoors if they exist.