Issue here is that what’s in a safe deposit box isn’t also being shared/distributed. It is locked away.
If, however, they made copies of the contents of a box and put it in other boxes … and it came out somebody used that for CSAM then there probably would be some kind of liability.
Besides CSAM there’s also copyrighted material, etc which section 230 kind of covers but even then gets tricky since there’s a duty to respond to DMCA takedowns in order to get safe harbor protections.
It’d be a little more like someone going to a Kinko’s (or whatever like that exists anymore) and using their copiers to copy CSAM.