Triangular slopes are a great boon when attempting to make natural terrain with various rolls and hills, since it's easier to keep them all aligned to each other. That said, it's not necessary, and slope sectors can have as many vertices as you want.
They're set up exactly like how CB did them. Linedef specials to slope the front/back floor/ceiling from its height to the height on the other side. I'll look into adding slopes by vertex things, but that'll come later. (And to be honest I'd like to figure out a good way to handle dynamically altering those before I implement them.)
FOF slopes aren't supported yet. If they happen at all, it'll be after everything else is done, because they're likely to take even more work than normal sector slopes did. And since I've seen a few people ask elsewhere, polyobject slopes aren't happening unless someone else feels like making that work. Those are already enough of a headache to set up.