22 Feb 2019:

I did not expect it to be that simple so I have to write about it. How to connect an outdated Linux laptop to the internet using an iPhone and its cable.

TL;DR (Debian Strech 9.x):

Plug the iPhone into the laptop via USB.

sudo apt install usbmuxd libimobiledevice
idevicepair pair
sudo dhclient

Done. That should be it. I like that sometimes opensource just works.

I used this arch linux article about iPhone tethering as source. Have a look there for more information.