First, to your question, there does exist a (very small) ban list. I don't know who calls the shots, but it's probably not the TCGO developers. It must be someone from TPCi, and they probably base themselves on the effects that different cards have had in the meta (paying particular attention to real-world tournaments) over time.
However, every card that has been banned has had a very good reason behind it. Lysandre's Trump Card was banned (to my understanding) because it effectively removed a victory condition from the game (decking out), although being able to endlessly recyle your own resources for infinite setup might have had something to do with it. FotGP was probably banned with its interaction with Shiftry (before, Shiftry was the card that was banned. That lasted until they realized that the problematic card was not Shiftry, but FotGP). It seems that TPCi decided that being able to evolve on your first turn (leaving aside the "Assention" attack, since that ends your turn) was a toxic mechanic, since it allowed stuff like T1 Trevenant item lock with a high degree of consistency and zero counterplay if the Trevenant player won the coin flip. Wally must have been banned for the same reason. Getting donked by multiple T1 Decidueyes musn't have been very fun either (speaking about FotGP here), but it was probably the other more problematic interactions that earned it the ban hammer. I do not know the exact reason behind the Hex Maniac ban, but it was probably too strong of a control tool.
For Brigette though, you can't really make a case. Stacking Stakatakas (pardon the lame pun) is nowhere close the "no counterplay" level that T1 Trevenant had. Against Stakataka-GX you can always drop a Silent Stadium, do too much damage for -30 to matter (consider that Choice Band "nullifies" the effect, leaving you at normal damage), use attacks that bypass effects, Guzma-snipe weaker targerts, use controll tools to keep your opponent from attacking (at which point no amount of damage reduction matters), etc. You have to consider that Brigette into 3 Stakataka-GX is a lot of resources commited for a measly -30 damage. Aether Paradise can do that, the upcoming Choice Helmet can do that, some pokemon effects and abilities can do that, and none of those consume your supporter of the turn nor 3 bench spaces. While your opponent only managed to set up 3 beefcackes that do nothing, you're setting upstuff like Macargo, Zoroark-GX, Malamar, Solgaleo-GX, Metalgross-GX, etc. Simply speaking, a mere 30 damage reduction scales poorly into the game if your opponent is instead getting stuff like literally choosing his or her next draw, drawing 2-4 extra cards per turn, having free retreat + effective inmunity to status effects, near infinite energy acceleration, etc.
I'd say that using Brigette for cards like Regirock-EX and Diancie Prism Star is more useful, but it doesn't fare a whole lot better. You're better off burning though your deck quickly with Sycamore, which can potentially give you your 2 Regirocks AND more things at the same time.
There really is no way to justify a Brigette ban no matter how you look at it. I doubt she has any real impact in tournaments either (although you'd have to confirm that with someone else because I almost don't play IRL). For any card to deserve a ban, it must break the game really badly in one way or another, and Brigette doesn't do that.
Edited by Sakura150612, 12 November 2018 - 09:38 PM.