Ophie
Salingerian Phony
As of when I'm typing this, Series 6 is underway in Sword and Shield Ranked battles. Each series has been 1 to 2 months long, so this one should end in October 2020, to be followed by Series 7. That one, if the pattern continues will be from November to December 2020.
In Generation VII, where they were known as Seasons, each one of them, from a certain point onwards, had certain rules. They were as follows, according to this list:
Season 7 (Nov 17 2017 to Jan 23 2018): All Pokémon allowed.
Season 8 (Jan 24 2018 to Mar 15 2018): 1 Pokémon allowed per team from the "Banned" list.
Season 9 (Mar 16 2018 to May 14 2018): You may only select 2 Pokémon per battle. No single battles.
Season 10 (May 15 2018 to Jul 10 2018): Inverse Battles (weaknesses and resistances are reversed).
Season 11 (Jul 1 2018 to Sep 3 2018): All Mythical Pokémon are allowed.
Season 12 (Sep 4 2018 to Nov 6 2018): All Legendary Pokémon are banned, including those ordinarily allowed. Mega Stones are banned.
Season 13 (Nov 7 2018 to Jan 8 2019): Banlist is inverted (Restricted Pokémon are the only ones allowed; all other Pokémon are banned).
Season 14 (Jan 9 2019 to Apr 2 2019): No hold items allowed except Berries.
Season 15 (Apr 3 2019 to Jun 17 2019): Metronome is the only move allowed. Sneasel line is banned.
Season 16 (Jun 18 2019 to Sep 3 2019): Top 30 Pokémon in usage are banned.
Season 17 (Sep 3 2019 to Nov 25 2019): Pokémon must be Level 1. Sonic Boom and Dragon Rage are banned.
There are some key differences this time around though. In Generation VII, every Pokémon ever made up to that point were allowed, giving you many more options than are available now. In addition, these were known as "Special Battles" and were optional. You could play Free Battles or Ranked Battles as normal, but you could opt for the Special Battle rules and play against other people under these same rules for even more rewards.
Since something like a Metronome-only competition would seriously mess around with people's ranks in Ranked mode in this game, I don't think they'll have something quite as extreme as that, or they will later split off these gimmick rules into something optional like Generation VII or a third option in Battle Spot (that is, there might be options for Casual Battles, Ranked Battles, and Special Battles).
Any guesses on what they'll do with Series 7 and beyond?
In Generation VII, where they were known as Seasons, each one of them, from a certain point onwards, had certain rules. They were as follows, according to this list:
Season 7 (Nov 17 2017 to Jan 23 2018): All Pokémon allowed.
Season 8 (Jan 24 2018 to Mar 15 2018): 1 Pokémon allowed per team from the "Banned" list.
Season 9 (Mar 16 2018 to May 14 2018): You may only select 2 Pokémon per battle. No single battles.
Season 10 (May 15 2018 to Jul 10 2018): Inverse Battles (weaknesses and resistances are reversed).
Season 11 (Jul 1 2018 to Sep 3 2018): All Mythical Pokémon are allowed.
Season 12 (Sep 4 2018 to Nov 6 2018): All Legendary Pokémon are banned, including those ordinarily allowed. Mega Stones are banned.
Season 13 (Nov 7 2018 to Jan 8 2019): Banlist is inverted (Restricted Pokémon are the only ones allowed; all other Pokémon are banned).
Season 14 (Jan 9 2019 to Apr 2 2019): No hold items allowed except Berries.
Season 15 (Apr 3 2019 to Jun 17 2019): Metronome is the only move allowed. Sneasel line is banned.
Season 16 (Jun 18 2019 to Sep 3 2019): Top 30 Pokémon in usage are banned.
Season 17 (Sep 3 2019 to Nov 25 2019): Pokémon must be Level 1. Sonic Boom and Dragon Rage are banned.
There are some key differences this time around though. In Generation VII, every Pokémon ever made up to that point were allowed, giving you many more options than are available now. In addition, these were known as "Special Battles" and were optional. You could play Free Battles or Ranked Battles as normal, but you could opt for the Special Battle rules and play against other people under these same rules for even more rewards.
Since something like a Metronome-only competition would seriously mess around with people's ranks in Ranked mode in this game, I don't think they'll have something quite as extreme as that, or they will later split off these gimmick rules into something optional like Generation VII or a third option in Battle Spot (that is, there might be options for Casual Battles, Ranked Battles, and Special Battles).
Any guesses on what they'll do with Series 7 and beyond?