I've received a reply from the Uber support. They told me that my account has been disabled in violation of the Terms & Services and that they are not gonna lift the suspension. This is confusing to say the least, because I have not violated anything, except if ordering a drive is a violation.
I want to address some things in this thread:
Why do you think this is because of GrapheneOS? This could also be because of your VPN, your PayPal account, your ...
I thought about this, and I cannot think of any other variable that could have had a negative influence on my account. I was not connected to a VPN, I was using mobile data from AT&T. I was logged into my private Google account on the Google Apps. My PayPal account never had any other issues. I've had this account for years now, and never had an issue with it like a negative balance or was I ever locked out of PayPal. Maybe my German phone number made some issues, because I requested a ride in the US but registered the Uber account using my German phone number. However, I was not able to find any supporting evidence for that on the internet.
Given all that, plus that in the exact time a notification "Google Play Services error" popped up - the only explanation I can see is that Google Play Services was the issue here.
Just use a bike or any other service
I will try out Lyft using a non-GrapheneOS based phone. A bike was not viable in my situation. However, the core problem is that you risk getting locked out of any account. With such a big player such as Uber, or any other company in their fields, risking getting permanently disabled because of your phone is no viable option.
You don't need the app, just use the website
Now if that's true, tell me more! I tried to use the web app, but it would always ask me to use the mobile app. If someone knows if you can also use the web app for Lyft, please tell me!
For my specific case I was able to take the public bus and a friend of mine will ride me in the next days, so I'm luckily good on that, but someone who doesn't have that kind of connection would be completely out of luck now.
So to wrap this all up, I am pretty sure that my Uber account got disabled because of GrapheneOS, because Google Play Services is not properly working, and Uber is not gonna lift the suspension. I may try to create a new Uber account using a new phone number and my credit card next year. I know that this is a violation of Uber's Terms & Services (and this would be a real reason to suspend my account) - however, if they suspend me for random reasons, don't even tell me the reasons and treat my support messages just like some generic help requests and just skim over what I write, then I don't care either and treat them like shit as well.
I will buy a new phone in the near future, and I am seriously consider not installing GrapheneOS on it anymore, after using GrapheneOS for a few years now. I am not happy about that cause I really like it and would love to continue using it, but again, risking lifetime suspensions is not worth it.