The answer is in the post Fizz linked, as well as some other things that was discussed in the link that explains why Venomoth ended up in RUBL in the BDSP formats.
Reason for Venomoth's Quickban
Venomoth does what it always does when it's broken- takes a single opening and turns it into a very easy sweep. If you don't have very specific counterplay options like Soundproof Bastiodon, you're going to be vulnerable to it. The council has deemed this, as well as its ability to turn even bad matchups on their head with good Sleep Powder luck, to be undesirable for the development of the tier.
Source
What Venomoth did was use Sleep Powder, Quiver Dance, Bug Buzz with Tinted Lens to sweep teams if the opposing team didn't have specific counterplay. Tinted Lens allowed Venomoth's Bug Buzz/Sludge Bomb to not be resisted by Pokemon who resist bug/poison moves. Venomoth would either run a Mono Bug Buzz set or a set with Sludge Bomb to beat a couple of the Soundproof Pokemon. These were the sets:
Mono Bug Buzz (Venomoth) @ Black Sludge
Ability: Tinted Lens
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Quiver Dance
- Sleep Powder
- Bug Buzz
- Substitute / Roost
Anti Soundproof (Venomoth) @ Black Sludge
Ability: Tinted Lens
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Quiver Dance
- Bug Buzz
- Sludge Bomb
- Roost / Sleep Powder
Counterplay to Venomoth
Venomoth's counterplay was rather specific. Soundproof Pokemon like Bastiodon, Exploud, and Mr. Mime were used to block Venomoth's Bug Buzz, though Mr. Mime and Exploud could still be hit by Venomoth's Sludge Bomb. Bulky special defense phazers would be used to get rid of Venomoth's boosts, but they had to be careful of Venomoth's Sleep Powder. Offensive Pokemon such as Moltres and Rhyperior would be used to pressure Venomoth out. Heck, Shedinja was an option because Wonder Guard makes it immune to Bug Buzz and Sludge Bomb.
Source
TL;DR Venomoth was quickbanned from RU due to having Quiver Dance, Sleep Powder, Bug Buzz, and Tinted Lens. This made Venomoth's counterplay limited, as Tinted Lens made Pokemon who could resist Venomoth's attacks unable to tank Bug Buzz and/or Sludge Bomb.
Hope this helps! :)
