How many games have released that required an SSD so far? I've seen quite a few that recommend it, but I can't think of any offhand that have absolutely required it. (I'm not worried, I've got two)
I really want to see DF or somebody immediately try to play it on a HDD to see what happens.. lol.
As a 'requirement', rather than recommended? Maybe only Star Citizen, that I'm aware of?
It's been a non-factor afaik until now - but now that consoles are getting there, for 'bigger' games I would imagine it'll start happening in the next couple of years.
HDD will, depending on the requirements for loading/unloading assets or accessing more information, likely either have huge loading times, and/or not work at all.
I'm kinda more curious if having an NMVE will help more or not (i.e. if their engine doesn't have a bottleneck on the system side for ssd speeds - IIRC Fallout 4 does have a certain bottleneck that can, or at least could, crash your game if you loaded too quickly)