Hm...food for thought.
Using all of the possible legal stages on this list, you have options that specifically hinder MK's ability to Perfect Plank. If these stages help MK more overall is another discussion, seeing how most MKs won't move post their favor 5 stages and thus we don't really have any evidence that he does better or worse on some of these stages.
Stage 1) Castle Siege. During the transition stages, MK cannot plank: no ledges to grab. During the Second Stage, MK cannot plank: no ledges to grab. That means for Slightly over 1/3 of the time on this stage, it's flat out impossible for MK to plank: 1/3 normal stages, and 3/3 of the transitions. There is also the fact that by planking, he runs the very small risk of being stuck under the stage and dying. Total Non-Plankable Time: Maybe 40% at best.
Stage 2) Delfino Plaza. There are three transitions where there are walk-offs, and thus no ledges to plank: the beach, and the two sidewalk sections. However, there is also that fact that during the flying sections, he can shark better than a great white. Total Non-Plankable time: Maybe 40% again. However, the trade off is not worth it for the vast majority of characters.
Stage 3) Norfair. Most of the time, this is a planking heaven, with a few select characters having a few more options to attempt to knock him away. However, there are times when lava comes from both the side of the stage, and the bottom of the stage. When this occurs, only one platform is free of lava, and you cannot plank safely, as you'll just touch the lava, go flying up, and then get punished. The lava walls can also force MK to switch sides that he's planking on, due to only Ike and Marth being able to hide in the lava. Total Non-Plankable Time: 5% at max, with a few times where he's forced to move away: maybe a dozen in any given 8 minute match? Not really worth it.
Stage 4) Port Town Aero Drive. The vast, vast majority of the time, there are zero ledges to grab. There is one transition with grabable ledges on the center platform, during a stop off of the track. However, there are platforms to both sides of this center piece, so characters can use a wide selection of moves to attempt to knock him off the one ledge. Or you know, just not bother as that happens so rarely. The moving platform is indeed sharkable, however his sharking ability is reduced, as he
must go on top of the platform every 5 jumps. This is not Delfino where he can go grab the ledge, and then continue to spam Uairs. You have a small moment were you can engage him normally. There is also the fact he can't even shark during the normal stops. Total Non-Planking Time: 95% at least, with nerfed sharking. This stage is in fact an option to use against MK, if you don't have a heavily limited character.
Stage 5) Rainbow Cruise. Only the ship is plankable. Then again, seeing as this is a mostly aerial stage, for most characters this would just be suicide. However, a few select characters I believe would have a chance. Wario for example. And then you get oddities like Mr.Doom beating MKs here on their CP with Ike. However, I doubt that would be consistent here. (Then again, Ike has both more range in the air, and faster aerial speed...hm...) Total Non-Planking Time: Around 80%. For a few select characters, this is an option.
Stage 6) Pirate Ship. This falls in the same boat as Rainbow Cruise, pun intended. MK can only plank on the left side for starts: the right side dips into the water during the rock phase, and would force MK off the ledge and into the water. Technically, you now have a chance to play keep away, as in MK away from that ledge. More importantly however, is the water itself below that ledge. As you don't instantly die for going there, some characters have a few more options to attempt to deal with his planking. Toon Link for example could get the right angle on his boomerang and bombs for once. Ike can just chuck aether at him. Total Non-Planking Time: 0%, though some characters are given more options to attempt to knock MK off of his ledge.
In a conservative stage list, you only have Castle Siege (Any character can use), Delfino (Almost no character can use), and Rainbow Cruise (A select few can use). MK can just ban Castle Siege if he wants to perfect plank, and largely not worry about anything disrupting his planking. Who in their right mind is going to pick Delfino? Or Rainbow Cruise unless they are Wario and feel like giving it a go?
In the BBR recommended rule list, you have Castle Siege (Any character), Delfino (Almost no characters can use), Rainbow Cruise (A select few), Norfair (A select few again, I could see Wario outdoing MK here), PTAD (Most characters could use this), and Pirate Ship (A select few). MK can ban PTAD, but is then forced to deal with no planking for about 40% of the time on Castle Siege. Or risking a character like Wario outdoing him on Norfair.
Again, take this with a grain of salt. I haven't seen any videos of MKs trying to be as gay as possible at PTAD. And as I'm sure ADHD in his trolling, Inui like ways will be happy to point out: I'm just a theorycrafter.
But honestly, if you aren't going to have a LGL on MK, you basically need a stage list that is at least a bit odd to make sure you have at least a chance to disrupt MK's perfect planking with the stage itself. Granted, you also need to actually win one of the games in the first place to even get your CP choice...
