Lukas, are you still have the same problem. I'm also running into the issue and looking to see if you found a solution. It honestly sounds like ios15 essentially bricked our iphones.

And I’m also getting the same issue. I have a 11 Pro. I woke up and all of a sudden The letter I is not working on my phone. I was on 15.1 and wasn’t even on the beta. I even tried downloading the beta profile and Installed 15. Two beta hoping that it would have addressed the issue. It did not. I tried restarting the phone, hard resetting, resetting settings. Nothing worked! I am literally using Siri to dictate in order to use my keyboard. I am taking the phone to the Apple Store but I doubt I’ll get a resolution but I will update this thread if anything changes. Also did the test where a whole column of my screen is not responsive.