If your device is running one of the older operating systems listed by the Pokémon GO Twitter account recently, your pokéjourney will be coming to an end unless you upgrade before October.
In an upcoming update to Pokémon GO in October, we will end support for Android 5, iOS 10, and iOS 11, as well as iPhone 5s and iPhone 6 devices. Trainers with devices not specifically listed here will not be affected and don’t need to take any action.
— Pokémon GO (@PokemonGoApp) August 31, 2020
To repeat: it’s only the OS versions or devices listed (Android 5, iOS 10, iOS 11, iPhone 5, and iPhone 6) that will not be supported after the October update. Everyone else is safe, for now. Pokémon GO fans knew this day was coming, since Niantic previously delayed ending support for older devices back in June after fan outcry. They needed time to figure out what OS versions the bulk of their users were actually using and it seems they’ve managed to do that.
To find out what operating system version your device is running, check under Settings > About Phone/About Device. Android 5 came out in 2015, and iOS 10 in 2016, so you’re probably due an upgrade by now anyway.
Niantic hasn’t specified exactly when the deadly update will drop in October, but you have been vaguely warned.