THis page mentioned Kubernetes API datastore -- https://projectcalico.docs.tigera.io/getting-started/kubernetes/flannel/flannel -- and it seems to suggest that Kubernetes API datastore is a replacement of etcd.
But I have not found more info on this "Kubernetes API datastore" and I do not know how to check if my current k8s instance is using "Kubernetes API datastore" or etcd as data storage component.
Could anyone tell me more about it?
So, it seems you must click on the last box of each "geek details" to have it show you the trade-offs: 
or you can view them in their source repo: https://github.com/projectcalico/calico/blob/master/calico/_includes/geek-details/datastore-kubernetes.md and https://github.com/projectcalico/calico/blob/master/calico/_includes/geek-details/datastore-etcd.md
The best tl;dr I can come up with is: if you have so many Pods that you end up overwhelming your kubernetes API with just Calico api chatter, then you can run a separate, dedicated, etcd just for Calico. Otherwise, pick the Kubernetes API because it's one less moving part to keep healthy
My experience is that it is a special circle of hell trying to keep an etcd cluster alive and healthy, so I would not wish a 2nd one upon my worst enemy (with the first one being the mandatory one backing the Kubernetes control plane)
