Do you know of actual (not hypothetical) cases, where you could "flip a switch" and run the exact same Kubernetes setups on 2 different cloud providers?

I run clusters on OKE, EKS, and GKE. Code overlap is like 99% with the only real differences all around ingress load balancers.

Kubernetes is what has provided us the abstraction layer to do multicloud in our SaaS. Once you are outside the k8s control plane, it is wildly different, but inside is very consistent.

Yes. I've worked on a number of very large banking and telco Kubernetes platforms.

All used multi-cloud and it was about 95% common code with the other 5% being driver style components for underlying storage, networking, IAM etc. Also using Kind/k3d for local development.

Both EKS (Amazon) and GKE (Google Cloud) run Cilium for the networking part of their managed Kubernetes offerings. That's the only real "hard part". From the users' point of view, the S3 buckets, the network-attached block devices, and compute (CRIO container runtime) are all the same.

You are using some other cloud provider or want uniformity there's https://Talos.dev

If you are located in germany and run critial IT infrastructure (banks, insurance companies, energy companies) you have to be able to deal with a cloud provider completely going down in 24 houres. Not everyone who has to can really do it, but the big players can.

Yes, but it would involve first setting up a server instance and then installing k3s :-)