A couple of questions:
Was the SSD requirement there at the start of EA?
No. it was not.
What happens if someone has the EA version but no longer has (or never had) an SSD? A refund after 3 years?
Ask Steam or GoG. I don't know.
Bonus question:
Has anyone come across an official explanation as to why an SSD is required?
Not official, but I think I am right about the Swap file needed for huge areas like Baldur's Gate itself.
It's the lesser of two evils - either you upgrade your RAM a whole bunch to some insane level like 64 GB or you get an SSD.
The SSD probably seemed more reasonable. This was likely the cost of not breaking Baldur's Gate up into transition areas.
I'm wondering what will happen on release when all the people who have no idea about the SSD requirement try to buy the game. Will there be some sort of warning or hardware check before payment is made?
Also: What if one has an SSD, e.g. used as the system drive, but doesn't want to install the game on that drive?
Then they will be fine until they aren't, because the game will run and then you will hit the big areas and then the game will start to really slow down.
Or maybe, they locked it so you cannot play without an SSD. That will really upset people.