While the new iMac Pro is not end-user upgradeable, Apple component supplier, OWC, did a teardown of the latest iMac Pro base model and found that the RAM and SSD can actually be upgraded if needed.
https://www.notebookcheck.net/iMac-Pro-teardown-reveals-upgradeable-RAM-and-SSD-but-it-s-not-for-the-faint-hearted.275628.0.html