Official Twitters:
Splatoon Japan (earliest announcements, in japanese however): https://twitter.com/splatoonjp
Nintendo Japan: https://twitter.com/Nintendo
Nintendo Europe: https://twitter.com/NintendoEurope
Nintendo America: https://twitter.com/NintendoAmerica
Nintendo UK: https://twitter.com/NintendoUK
Organization Twitters:
Ink.TV: https://twitter.com/InkTVSplat
EndGameTV: https://twitter.com/EndGame_TV
IkaSportsTV: https://twitter.com/IkaSportsTV
SquidBoards: https://twitter.com/Squidboards
Luti: https://twitter.com/InkLeagues
ESHUB Splatoon: https://twitter.com/SplatoonESHUB
IkaFeed: https://twitter.com/IkaFeed
Message Snek#9658 on Discord for adding Twitters to this list