IDC. Not many people used it. It didn't really last for long and it was still banned. Just mentioning this because you shouldn't use popularity or lack of popularity as a reason to ban or not ban something.
note: hotly disputed.
If anyone thought IDC should have been legal, they were stupid. With IDC, you
literally could not touch your opponent because your opponent was
literally invisible, invincible, and untouchable. No question about it. Obviously that's going to be banned, because no character or person with any amount of skill can get past it.
Planking's different. Planking doesn't give you clear-cut invincibility, and you can technically approach and attempt to hit the planker. The problem here is that if you go to attack, you're putting yourself in a position in which you'll easily get gimped and die, or at least suffer a lot of percent. But it's still physically possible to approach a planker, unlike IDC.
Plus, some characters do have options to stop planking. Pikachu can use his neutral B, Diddy can arc bananas, etc. Planking isn't invincible for every character. However, Planking does destroy at least half of the cast, and they have no viable options to stop it other than putting themself in a disadvantageous position.
Which is why some can argue that planking shouldn't be banned, because it's still physically possible (although stupid and disadvantageous) to hit your opponent, and you're stalling just as much as he is if you aren't trying to approach. And some argue that planking should be banned, because it shuts down way too many characters and there's nothing you can do about it for them, and stalling is already banned, so planking should be too.
There really is a lot of debate about planking, whereas with IDC it was obvious that it had to be banned.
Personally, I hate planking and think it's stupid, but I don't think it should be banned at this point. It really is too early to vote. Not enough people plank to the point where it's even a valid problem at any tournament. That combined with the fact that some characters can get past it, and possibly even more when more people experiment with getting past planking, basically secures my thought that it's way too early to do anything about planking.