[prng2021]

In the early days that was true. K8s has had many options for stateful containers for a while though.

https://kubernetes.io/docs/concepts/storage/

[prmoustache]

we are talking in a context where you would spin up a new cluster whenever you want to upgrade kubernetes version.

In that case you don't want to migrate application/user data so you are kind of forced to keep DBs and filesystems outside.