In previous games, the only Pokemon that were banned from VGC were the "True" Legendaries, meaning all of the box art legends plus some. These include
Mewtwo
Mew
Ho-Oh
Lugia
Celebi
Groudon
Kyogre
Rayquaza
Jirachi
Deoxys
Dialga
Palkia
Giratina
Regigigas
Manaphy
Darkrai
Shaymin
Arceus
Reshiram
Zekrom
Kyurem
Victini
Keldeo
Meloetta
Genesect
Xerneas
Yveltal
Zygarde
Diancie
Volcanion
Hoopa
Solgaleo
Lunala
Necrozma
Magearna
Marshadow
Zeraora
Meltan
Melmetal
Zacian
Zamazenta
Eternatus
Zarude
Tell me if I am missing any!
However in Series 6 of Sword and Shield VGC 2020, they banned a large selection of Pokemon. These Pokemon were
Venusaur
Gyarados
Porygon2
Tyranitar
Torkoal
Hippowdon
Magnezone
Togekiss
Excadrill
Whimsicott
Incineroar
Mimikyu
Rillaboom
Cinderace
Indeedee
Dragapult
Series 6 is over now so the above Pokemon are usable now
[Source][1]
[1]:
https://www.millenium.gg/news/19788.html