The API will almost certainly be the modern one, because the old one was highly exploitable and allowed pseudo-botting. Will probably have the same access/limitations to information for that API, which should mean modern combat logging. I think you can probably assume things like blink will be fixed by modern infrastructure.
The more interesting question would be stuff like say, Vanish, which was highly unreliable In Vanilla and BC, and then changed to work more reliably in Wrath by actually altering how the spell worked. So do you leave it broken and unreliable or do you implement a version of the spell from several years later so that it works as intended?