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 How to connect an outdated Linux laptop to the internet using an iPhone and its cable.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.