Something I'd like to clarify before this gets any further: there's a fundamental difference in OU between, say, Shadow Tag Gothita and Moody Whimsicott; one of them is banned, as Shadow Tag isn't allowed, while one of them is illegal, as Whimsicott can't
get Moody. Specifically, this means that one of them
can be allowed in the future should the banlist change, while the other requires interference from Nintendo.
I don't know what's the best policy for other metas besides Sketchmons but regarding Sketchmons, I do NOT agree about blanket banning the moves on original abuser, simply because it doesn't follow the basis of the meta. The basis of Sketchmons meta is the move "Sketch" which makes you learn any one of the illegal moves. So it makes more sense if the banning policy also follows the basis of the meta, in this case, ban the move from being Sketched, NOT just blanket banning the move.
Regarding AAA, BH, etc. I don't really know what's the best decision for them since there's no "Sketching Abilities" so this discussion make sense BUT in Sketchmons, there is a mechanic of the meta in that can be applied to the banlist, so I think it's best that Sketchmon's bans (especially move bans) also follows the basis mechanic of the meta.
You also banned ability/move from the main natural users of the move/ability. While this is fine if the abuser is powerful enough, it leads to bans that don't need to happen. Sometimes the natural user is fine to have an ability/move on, as it is not powerful enough on its own. And while this is true, going through a laundry list of over 800 Pokémon to determine which one is broken with a specific move is simply insanity in a form of work, and is simply not feasible for a number of reasons. (It helps giving the natural users that aren't broken an advantage, as well. Klinklang would suck in Sketchmons without Shift Gear.)
Checking the natural users to see if they are okay is a better decision, and one that has worked for a long while now. It is why Kangsaskhanite is still banned in AAA; Mega Kangashkhan is too powerful with Parental Bond. It is also why Shadow Tag Gothitelle (and the whole line, just in case someone is crazy enough to use them) is also not allowed in AAA; we use the most powerful form of the natural users to determine if an ability is broken, and then the banning policies are put in place.
I'm going to address these together, as I feel they follow the same flawed argument that I have an issue with; if either of you feel your points are misrepresented here, please let me know.
My issue with all of your suggestions is that it's a complex ban. Sketchmons and Stabmons add a mechanic to the game. It's not a list of unbans like you're unbanning and then rebanning Shift Gear Mega Metagross, it's something that, for the purposes of the OM, fundamentally changes that sort of legality so that Metagross can
learn Shift Gear. This is fine. However, when you restrict based on that it's the problem. For Sketchmons or Stabmons, banning a move from being sketched is exactly the same as banning it from being gained via TM's, or event, or eggs: both are simply a way the Pokemon can gain the moves. It's a change in legality rather than an in battle ban; the teambuilder doesn't reject it because it's carrying Shift Gear, but rather because it obtained Shift Gear via Sketching/Stabbing, which is in opposition to everything Smogon has done with tiering.
With AAA, the same issue comes into play. AAA as a method for gaining access to an ability is no different than hidden abilities or except in breadth. The issue can't be powerlevel, as Dugtrio, say, is blatantly more powerful with Arena Trap than Magikarp or Sunkern, and even if you weren't sure where to draw the line they would be on the other side. Instead, AAA has chosen to simply cut off one method of gaining an ability rather than the ability as a whole.
Again, my apologies here: I'm going to be splitting up your post to better answer it.
It makes sense in OU for Shadow Tag to be banned on everything because the community has decided the ability is uncompetitive wholesale. If Shadow Tag is banned, that's the end of it. OMs are by definition not standard, but we build a lot of our banlists off of OU (or Ubers/DOU/LC/etc) because it's just easier to manage them that way, which means in essence that OMs borrowing the OU banlist are "Standard+."
Now, with a metagame like Sketchmons, the mechanic allows for one free Sketch per Pokemon. This mechanic is technically just a change to access (STABmons, AAA, and BH do this too). In your example, Sketchmons hasn't made Shift Gear any better, it's just expanded access to the move. What the mechanic has done is made individual or groups of Pokemon better because they can now use the move. When Sketchmons decides to ban Shift Gear on only non-native learners, it is staying true to the mechanics of the metagame--it's removed that access.
This is where the definition above kicks in: our banlists for almost every OM starts at OU. Deciding that Metagross can learn Shift Gear doesn't effect banlist at all, it just changes legality. We may ban or unban Pokemon (or in a few cases items/abilities), but our metagames by themselves don't, and no Pokemon in Sketchmons is banned by ou; were the Pokemon to get those moves naturally, as of right now they would be perfectly legal.
When Sketchmons does that, therefore, they
aren't doing that. As shown by the precedent
here, we
don't get to ban things based on where they came from, or because we would prefer the metagame. At the risk of repeating myself overmuch, we can't prevent Pokemon from getting the move via sketch any more than we get to prevent them from learning it via event or by move tutor.
Shift Gear may be broken in principle, but that's not Sketchmons's job to determine. What Sketchmons can do, however, is determine whether Shift Gear is broken on other Pokemon besides Klinklang, and if too many of them are, curtail that access. On the topic of Shadow Tag, it's universally agreed that it's uncompetitive on everything in OU. There was no need to ban Gothitelle but save Gothita. Same with Moody, self-evasion, Swagger prior to Gen 7, etc. OU hasn't determined that Shift Gear alone breaks any Pokemon with the move, and thus the baseline is set. Now its Sketchmons's job to apply its mechanic and trim back the excess.
Now, I'm aware that something like BH has taken an all-or-nothing approach to bans, but in a metagame as free as BH and that has no baseline banlist to adhere to, this is acceptable. So long as the other ladders are consistent with whatever method they choose (e.g. avoiding something like banning Shift Gear universally but allowing Belly Drum on native learners), I don't see the problem with the variability.
In regards to complexity, I don't see how such a ban can be considered complex. As I've already shown, Sketchmons is only concerned with move access. OU, however, doesn't have this problem and so when it bans Shadow Tag, it doesn't have to worry about making exceptions. Now, your Swift Swim Magikarp example is weak. Last gen, the Drizzle + Swift Swim ban would have prevented a Swift Swim Magikarp on a team with Drizzle regardless of how effective it is. The ban is consistent. There's nothing complex about saying "Access to Shift Gear is cut-off to non-native learners." Standard players are already playing with Shift Gear limited to Klinklang; why suddenly can't they understand the move is too good to be allowed on everything else?
Again, tying this in; it's my opinion that Sketchmons
shouldn't be able to break Smogon policy and do that. You don't ban something on 95% of Pokemon because it's broken on 60% of Pokemon; either <Pokemon with Shift Gear> are too strong, and thus the move needs to go entirely, or it's not and we should remove broken users This halfway
legality change, however, is far worse than either decision.
In terms of complexity, I don't see why it would matter if Sketchmons bans are consistent; they're still complex. It would be consistent to ban Ubers from having moves over 60 BP, but wouldn't make it any less stupid. While obviously this isn't that bad, the only difference is in scale.
The last part of this is much more compelling. While I would disagree that my Swift Swim Magikarp example is weak (this is where I was going with it; despite it not being broken, it's still banned because the strategy was). DrizzleSwim, however, was
also a complex ban. I also dislike it for that reason, but because of that I am willing to accept them should they be direly needed for metagame balance. However, I have seen no evidence that that is the case - if Shift Gear Magearna, Thousand Arrows Zygarde, or Arena Trap Dugtrio is a powerful, necessary balancing factor, please support that - so as is I see no reason why we should go complex, particularly on a scale this large.