Update from Niantic
Update: Niantic has announced that they are aware of the issue and will be working towards a fix in their next update.
Trainers, Pokémon GO will be supporting iOS 14 in its next update (0.187), which will be coming out soon. To continue playing Pokémon GO on your iOS device, we recommend not upgrading your device to iOS 14 until Pokémon GO version 0.187 is available to you.— Niantic Support (@NianticHelp) September 15, 2020
We recommend that you still wait to update, but it's nice to know that a fix is on its way!
Players have been actively reporting issues with Pokemon Go after updating their Apple devices to iOS 14, which is confirmed to release on September 16th.
Niantic has not posted any information in regards to these issues yet, though it's expected that Pokemon Go will see an update in the coming days in order to add iOS 14 support. In the mean time, it's recommended that Pokemon Go players wait to update their devices.