You can access the data in an encrypted backup, which you can request from an iPhone from Linux using the open source libimobiledevice: https://github.com/libimobiledevice/libimobiledevice

Here's an overview of how to remove the various layers of encryption (starting from the backup password): https://stackoverflow.com/questions/1498342/how-to-decrypt-a...

And how to do it if you want to access the WhatsApp chat database: https://yasoob.me/posts/extracting-whatsapp-messages-from-io...

Also some Go tools to inspect iOS encrypted backups https://github.com/dunhamsteve/ios