Aug 2020
Effective Date: Aug 28, 2020
Format: Standard
These were the first cards banned from a Standard format in more than 5 years, following the ban of Lysandre's Trump Card in 2015. But, what made this ban more interesting than any before it, was that Pokémon re-created the cards with alternate art & alternate text at a later date. See reprinted erratas below:
Feb 2019
Effective Date: Feb 15, 2019
Format: Expanded
Lusamine was later un-banned from the Expanded format on Mar 5, 2021, along with Wally (ROS).
Aug 2018
Effective Date: Aug 17, 2018
Format: Expanded
Wally (ROS) was later un-banned from the Expanded format on Mar 5, 2021, along with Lusamine (CIN).
Aug 2017
Effective Date: Aug 18, 2017
Format: Expanded
Shiftry (NXD) was un-banned from the Expanded format on this date.
Shiftry
Effective Date: Sept 1, 2015
Format: Expanded
Shiftry (NXD) was later un-banned from the Expanded format on Aug 18, 2017 with the announcment of Forest of Giant Plant's ban.
Lysandre's Trump Card
Effective Date: June 15, 2015
Formats: Standard & Expanded
While not the first card to ever be banned from Pokémon TCG play, it was the first card to ever be banned by Play! Pokemon. (WOTC was in charge of both Slowking & Sneasel's bans)
This card's ban announcment occured on June 2nd, just 32 days before the US National Championships. Thus, resulting in players having to work their way through a brand new format before the event.
Slowking
Effective Date: Jan 1, 2003
Format: Modified
This is an incredibly interesting card, because it was mistranslated from the original Japanese text. Slowking's Poke-Power was intended to work only if Slowking were to be in the Active spot. However, in English, it's read as if it can work whether Active or Benched! This wasn't caught until the end of 2002, and rather than errata'ing the card, WOTC outright banned it from organized play. Many retro players advise that you play Slowing (N1) as intended by Japanese card designers, with the errata, in any Neo-legal format.
Jason Klaczynski goes into further detail on Slowking (N1) in his blog, read here.
Sneasel
Effective Date: Sept 1, 2001
Format: Modified
This card dominated the format upon its release in Neo Genesis. As WOTC announced the first ever Modified Format, (Team Rocket - on), they announced that Sneasel (N1) would be the first legal card to ever be banned in Pokémon TCG history along with the start of the new format.
Other "Banned" Cards
Blaine's Quiz Show
Formats: Standard & Expanded
This card was only banned for the duration of the 2019 Pokémon World Championships. Reasoning: it had the potential to create complications in a multilingual environment. Its ban was lifted after Championship Sunday.
Imakuni's Doduo
Literally printed on the card:
"This card cannot be used at official tournaments."
Obviously this card was never legal for tournament play, but it's on this page as a reminder that there have been many cards in the TCG's history that have had similar text written on the card. This being one of the most recent.
Wormadam
Effective Date: June 26, 2008
(Was banned for this day's event only)
Format: NA Professor's Cup
This is a silly instance in Poké-history, Wormadam was a Rare card in DP: Secret Wonders. However, when reprinted in POP Series 7, it was printed as an Uncommon card. Why does that matter? Well, the weekend of the US National Championships, the annual Professor Cup was set to take place. Their formats have always had twists to them, this year specifically, Rare cards weren't legal. Since this card was intended to be Rare, but reprinted as Uncommon, it was banned from this particular event. (It dominated the 2008 European Professor Cup, before the "misprint" was discovered as game breaking.).
Birthday Pikachu
Effective Date: September, 2000
(Banned upon its Release)
This card was banned from organized play before player's could even get their hands on it, for the specifically obvious reason, that no player could prove if it were to be their birthday.
Ancient Mew
Effective Date: July, 2000
(Banned upon its Release)
This card was banned from organized play before player's could even get their hands on it due to its unique text design. It was too cryptic to be considered legal for organized play, with the thought process of some players claiming to know what it could do, while possible that not even Judges would be able to rule correct or not.
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Card Text: Mew 30HP - Psychic type. It has 1 attack that does 40 damage which requires 2 Psychic Energy. Weak to Psychic (x2), no Resistance, Retreat Cost of 2.