Did Sunfire Cape use to scale with AP?

No.

No, but I think it scaled with magic penetration, as it dealt magic damage (still might, haven't checked).

never.

That would make it kinda useless as a tank item. It would require AP to be useful, but it's a tank item.

It never scaled with AP. It used to not scale at all, even with level, until Riot decided that the 40 magic damage per second was deemed too strong at early levels, so Riot made it to how it is today.

Also, what DrCyanide said, since it deals magic damage, it's damage will work with your magic penetration.

Not, but I think magic penetration affects the damage.

No, the only changes to it really was having the damage scale per level so it was weaker when bought early, and making the passive unique as several years ago you could just buy 5 of them and they stacked.

It never scaled with AP.

It didn't even scale at all before.

BUT, way before, at the start of league, you could buy multiples and stack the passive aura.

And also Eve wasn't detected when near champions. This made for hilarious stealth champions walking around champions and burning them slowly to death.