Niantic have announced a change slated for November 16, 2023 that will affect those of you who use Pokémon Trainer Club to log into Pokémon GO. Most notably, users may need to log back in and will receive an unspecified reward for doing so. Niantic also recommends adding another login method — such as logging in with a Google account — so that you do not lose access to your account in the event that PTC or another method becomes unavailable for whatever reason.
Their full blog post detailing this change is below:
Starting on November 16, 2023, there will be a change to the way that you log in to Pokémon GO using Pokémon Trainer Club credentials. If you log into Pokémon GO with a Pokémon Trainer Club account, you will still tap Sign-In with Pokémon Trainer Club, however you will notice an updated login screen.
As a result of this change, any Trainer currently logged in to Pokémon GO using Pokémon Trainer Club credentials may be logged out and will need to validate to regain access to Pokémon GO.
Ahead of this update, we recommend you take note of your Pokémon Trainer Club username and password, keeping in mind that your Pokémon Trainer Club username may be different from your Trainer Name in Pokémon GO. Once this change has been completed, Trainers who have a Pokémon Trainer Club account will be able to receive a reward for validating your account. More details on this item bonus will be available soon.
Also, if you do not have an additional login method attached to your Pokémon GO account, we recommend adding one so you don’t lose access to your account in the event that one particular login service becomes unavailable. After this change has been completed, any Trainers who do not currently have a Pokémon Trainer Club account linked to Pokémon GO will be eligible to receive a reward for linking one. Learn how to add an additional login method to your account here.
If you have forgotten your Pokémon Trainer Club username and/or password, you may recover them here. If you need further assistance, please contact The Pokémon Company International’s Customer Support team here.