How do iPhones handle partial vacuums? I've used a vac chamber to speed up helium off-gassing from poisoned rubber gaskets in the past.
For the electronics I would expect it’s fine. The oscillators should be sealed. Whatever is inside the MEMS cavity would be okay. Now, all the mechanical stuff presumably expects some sort of normal atmospheric pressure, so YMMV on speakers, haptics, and any of the camera OIS. It’s all water sealed these days, though.