Pokémon GO was recently updated globally to version 1.85.4 and 0.115.3 on iOS and Android, respectively, implementing a new QR Codes feature, various bug fixes, a new UI for notifications and more. Unfortunately, the latest update also implemented a new issue that causes the Pokémon GO app to crash upon loading on iOS. The official announcement from the development team at Niantic is as follows:
Trainers that have upgraded to iOS version 1.85.4, we’re aware of an issue that causes the Pokémon GO app to crash upon loading. If you are experiencing this issue, please try to reinstall the app.
Source: Niantic Support @NianticHelp