Niantic announced on their Pokémon GO Twitter feed that the storage for Pokémon GO has been expanded to 6050 for Pokémon and 5050 for Items. Players still have to purchase the upgrades to unlock the space.

Pokémon GO Storage Expanded

Trainers, we have increased max storage capacity for both Pokémon and items by 450, with an additional 50 to follow at the start of Pokémon GO Fest 2022 in your local region!

Source: https://twitter.com/NianticHelp/status/1532559068715433984