Good question! IMHO, it's a spectrum, of course, not a binary concept.

But if we have to define a criteria... I guess, integrated just enough so it can't be trivially removed, making it more of a "body part" rather than a "tool".

Point is, it'll certainly spark a discussion and re-evaluation of what's "fair", potentially shifting the consensus from somewhere around the current "glasses are fair game, but a programmable mouse is not" to somewhere more accepting of differently-abed individuals.