Server MacRumors.com found interesting information about new, just released 5K iMacs. If you purchase option with lower storage – under 4 TB – your SSD is soldered to Logic Board. If you purchase 4 TB or 8 TB storage option, there is a flash storage expansion board, which is attached to a connector on the Logic Board. This is interesting. Currently, I have older CTO 5K iMac Late 2015 with pure SSD and I can replace my SSD or even to add another one into HDD slot used in Fusion Drive versions, though I know I will be limited on speed and I need special tools, like ones from OWC. In real World, it means that SSDs used in configurations under 4 TB are not replaceable without new Logic Board.
If your Logic Board get damaged, all your data are lost. You will need to make regular backups. As was discovered, it’s because of hardware encryption, as Apple said in internal documents.
New 5K iMacs feature a T2 chip. Soldering SSD drives to Logic Board is another shot on Macs’ modularity. Even Mac’s Pro main SSD disk must be replaced by Apple, because of T2 chip. It may be good for security, but it brutally kills modularity of already almost non-upgradeable Macs. Hopefully, you can still easily replace RAM modules in 5K iMacs.