"We are hoping this will be an easy transition"
If you're an Android user and are still playing Pokémon GO regularly, as of next week, Niantic will be changing how trainers with these particular devices redeem "offer codes".
After 13th February 2024, Android devices will be "unable to redeem offer codes within the app". From here on out, all users will be required to redeem their offers via Pokémon GO's web store.
"We are hoping this will be an easy transition for Trainers and will provide a one-stop shop for everyone! If you have any additional questions about the offer code redemption process, please visit the Niantic Help Center for more information."
It's also mentioned how trainers with a Pokémon Trainer Club account "can redeem offers by linking their Pokémon GO account to another provider" which is then used to log in to the store.
Keep in mind, in-app offer code redemption has only previously been available on Android devices, with users of iOS devices already required to use the website to redeem these codes. If you want to obtain these special codes, keep an eye out for Niantic's special events and sponsored partnerships going forward.