You can self-install Knative components to any Kubernetes cluster. This means, your operator managing the cluster needs to keep the installation up to date and run reliably.
There are cloud providers that offer Knative-as-a-service, or manage it for you in your cluster:
Google Cloud Run for Anthos/GKE: A managed Knative installation in your GKE clusters.
IBM Cloud Kubernetes Service: A managed Knative installation in your IKS clusters.
Red Hat OpenShift Serverless: Knative installed and managed by OpenShift on your OpenShift clusters.