Ultra League and its Premier Cup have left Pokémon GO with Master League returning along with a brand new cup: the Retro Cup. Here are all the details on these cups which are currently live in GO Battle League.
Master League is back with the standard Master League and Master League Classic. The standard Master League allows any Pokémon regardless of how high the CP is. Master League Classic puts a new limitation: Pokémon that have been powered up with Candy XL cannot enter. The Retro Cup is entirely new and has the Great League CP limit of 1,500. It only allows the first fifteen Pokémon types, which means that any species with Dark, Steel, or Fairy-typing are excluded.
The Master League, Master League Classic, and Retro Cup are now live in Pokémon GO's GO Battle League and will be available until Monday, May 17th at 1 PM Pacific. Unfortunately, we are still being robbed of the excellent Master League Premier Cup, which (I believe, anyway) offers far more variety than Master League Classic, as there aren't many meta-relevant species that trainers would've been able to power up with Candy XP by now anyway.
This Master League and Retro Cup run will be the last leg of official league-specific gameplay during GO Battle League Season Seven. Pokémon GO will open up all leagues (including Great League, Ultra League, and Master League) starting Monday, May 17th. This will run for just one week and then ranked play will end on Monday, May 24th at 1 PM Pacific. Once ranked play is over, an unranked Kanto Cup will be available in Pokémon GO until May 31st at 1 PM Pacific. It is expected that Niantic will soon announce GO Battle League Season 8 along with the next overall Season of Pokémon GO gameplay that is expected to begin on May 31st.