Think it's needed as Rare Mark Shinies and Shiny Dudunsparce-Three/Maushold-Three have the same rarity (1/409,600)
#Banning Rare Mark Shinies
junior gate
void frigate
I agree, I believe the rare mark is even rarer at 1in1000? I don't know how title power affects this, but a mod responded to me in #server_questions saying that the other marks are fine as while they are usually 1in100, the title power level 3 which usually comes with the shiny sandwich increases this chance of finding marked Pokémon. However the rare mark I think is still too rare
viral lynx
Have you considered any contextual differences between the pokemon you are talking about outside of the odds? The odds for 3-seg and 3-mouse only apply to those pokemon - the "rare mark" can be obtained on any pokemon which can have the mark (nearly all pokemon available in wild encounters).
The chance of someone getting the rare mark on a random pokemon they are willing to trade seems at least on the face of it to be easier than a single species which must be hunted in discrete circumstances to obtain its rarer form.
Tradebans typically represent a way of preventing hacked versions of an extremely rare Pokemon being distributed wholesale throughout the server and community informed by statistics but not solely based on the numbers 
void frigate
Have you considered any contextual differences between the pokemon you are talki...
This is an interesting addition to the discussion, however there are ways of forcing only certain Pokémon to appear, or increasing the number of a certain Pokémon so it is effectively the only Pokémon appearing. An example of this is dundunsparce at the lower levels of area zero, in which the difference would be nothing between the rare mark and the dundunsparce chance in that case. Except the rare mark is actually 1in1000 instead of just 1in100. Therefore a shiny and rare marked would be more rare than a shiny 3 segment dundunsparce. However I am not sure how much title power affects this, as per my above message. If title power increaseses odds of the rare mark by more than 10x then it would even out and become more common by comparison
But yes, it seems that having a rare mark is not too desirable for people, and so is not a point for banning in that perspective since there isn't much demand. However a rare marked shiny is incredibly suspicious if offered for trade since the odds are so low and it is so easy to add this mark to hacked pokemon
viral lynx
Again, the context of hunting is important. Even if the odds were similar, you're talking about solely hunting one thing. For anyone who is not dedicated to only hunting that one thing, it is impossible to obtain the form. Marks are available on (most if not all) wild pokemon. It is entirely conceivable that despite the odds, people end up with shiny rare marked Pokemon from shiny hunting pokemon (especially in outbreaks with good sandwiches). We're not comparing apples with apples when only looking at the statistical chance in isolation.
Suspicion can be worked through - a trader can take that into account when they trade, and make their judgement call on aspects like the trader's profile, their behaviour, the pokemon and its background, and any other surrounding circumstances.
A tradeban removes that due diligence need altogether for pokemon that would otherwise be popular for trading (shiny 3-seg and 3-mouse) - just removes the issue altogether. If the 3-forms are removed from the tradeban list at any time, then a trader can revert to the above process for checking any suspicion in each case
All of this is just one person's (my) opinion, my point being that the Mod team's decision to tradeban or not is a complex decision made from an assessment of many different aspects. While statistics are the best place to start, the context is incredibly important
void frigate
Again, the context of hunting is important. Even if the odds were similar, you'r...
I have been convinced, it makes sense now! This is a really thoughtful and thorough explanation, thank you for your time😊