The Pokémon Company announced on the official Pokémon website the 2021 Season Pokémon TCG Format Rotation.

2021 Season Pokémon TCG Format Rotation

Find out which cards will be permitted in the 2021 Pokémon TCG Championship Series season.

At the beginning of every Championship Series season, Pokémon Organized Play removes older expansions from competition in the Standard format, with the goal of maintaining a healthy competitive environment. This rotation challenges existing players to create new strategies and enables new players to get involved in Play! Pokémon events using the most recent releases. Events taking place after August 28, 2020 will use the 2021 season Standard Format.

The Standard format for the 2021 season will consist of the following expansions:

• Sun & Moon—Team Up and any expansions released afterward
• Black Star Promo cards numbered SM158 and up and SWSH001 and up
• Cards from the following products are also legal:
  • Hidden Fates (excluding all cards from the Shiny Vault subset other than Lycanroc-GX SV67/SV94)
  • Pokémon TCG: Detective Pikachu
  • Cards from McDonald's Collections will be legal in the Standard format only if they're functionally identical to a card that is currently legal in the Standard format. New expansions become legal for tournament play on two weeks after they are released.

All cards from the Sword & Shield Series have a letter in the bottom-left corner called a regulation mark. In the future, this mark will determine which cards are allowed in tournament play. For the 2021 season, all cards with the “D” regulation mark are legal to use in tournaments. New cards with this regulation mark still must follow the appropriate waiting period to become tournament legal (typically two weeks).

The Expanded Format for 2021

The Expanded format will remain unchanged for the 2021 season. Players should double-check what format is being used before choosing a deck to bring to their local events.

The Expanded format for the 2021 season will consist of the following expansions:

• Black & White and any expansions released afterward
• Black Star Promo cards numbered BW01 and up, XY01 and up, SM01 and up, and SWSH001 and up
• Cards from the following products are also legal:
  • Generations
  • Double Crisis
  • Dragon Vault
  • Shining Legends
  • Dragon Majesty
  • Hidden Fates
  • Pokémon TCG: Detective Pikachu
  • Black & White Trainer Kit and any Trainer Kits released afterward
  • McDonald's Collection 2011 and later

New expansions become legal for tournament play two weeks after they are released.

Please note that several cards are currently banned from the Expanded format. You can find the list of banned cards here.

Source: https://www.pokemon.com/us/pokemon-news/2021-season-pokemon-tcg-format-rotation/