The Kubernetes team chose etcd specifically because they were trying to replace Borg's master/slave database at Google. Nothing about Kubernetes requires etcd; the team was trying to solve a Google-internal problem with it (and in the end, didn't gain traction within Google.) k3s uses sqlite by default which was an option at the time, other clusters today use PostgreSQL.
Have you looked at the etcd keys and values in a Kubernetes cluster? It's a _remarkably_ simple schema you could do in pretty much any database with fast prefix or path scans.
There are many forms of Kubernetes that are not using etcd that are certified by the CNCF as conformant (https://www.cncf.io/training/certification/software-conforma...). The version in the kubernetes/kubernetes repository is a reference implementation, intended to be customized by the community.
Have you looked at the etcd keys and values in a Kubernetes cluster? It's a _remarkably_ simple schema you could do in pretty much any database with fast prefix or path scans.