I think this statement is not as precise as it should be. I'm running docker produced images in a k8s cluster and had to google what you are talking about here.
https://kubernetes.io/blog/2020/12/02/dont-panic-kubernetes-...
"Docker-produced images will continue to work in your cluster with all runtimes, as they always have."
Dockerfile is a very common file to see in projects, and I'm thankful when I see it.
Although a pattern I commonly see these days with a lot of OSS projects is that oftentimes they provide a docker-compose.yaml even though no one at any reasonable scale is running Docker Compose in production. This is simply because Kubernetes setups are complicated, weighty, and heterogenous, and often not running on your local machine and Docker Compose is a great way to do a hello world style demo of your container-orchestrated app (because everyone still uses Docker Desktop).
Not sure about this, Docker is still widely used in CI systems and local development in my experience.
It's also widely used in Opensource projects for "reproducible builds"