The iMac Pro does have slotted RAM, they showed it off multiple times in the keynote. But like this 4K model, the display has to be removed to access it, along with the processor and SSDs.
I said upgradable, not slotted. And no, having to remove the display and void the warranty isn't "upgradable" for a mainstream user. And this model (which is 5k, not 4k) has a memory door on the back and Apple lists the model as user accessible. That's my whole point, when the "consumer" 5k 27 inch has a memory door, why not do the same on the "pro" model?
https://support.apple.com/en-us/HT201191#install1
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It's upgradeable, but some work is involved ... and probably a voiding of the warranty: http://ifixit.org/blog/9182/2017-imac-upgradable/
That's not the iMac pro (which doesn't ship for months), that's the 2017 21 inch iMac.