Once again, the 'myth' of hermetic resistors vs humidity, yes hermetic is essentially immune to humidity at a very high price point. Nobody of significance, Keysight, Fluke, ect., are using hermetic resistors in the LTZ circuits because it isn't necessary. But if you want to spend a whole lot more money on hermetics to find this out, be my guest, it isn't my money you're spending. There are many LTZ circuits that have gone into far more difficult environments than a 3458A and they don't use hermetic resistors, they work just fine. Yes some resistors are very sensitive to humidity but well made resistors should be no problem in LTZ circuits.
Check your circuits, you've likely got other factors that are out weighing humidity effects and if humidity is causing a problem, you should take the time to find out just what is being sensitive to it and not just grabbing at straws.