No indication is given in docs that controller (master) nodes should be rebooted serially 

    4.13 - 4.16

    It is our understanding that with ETCD and RAFT cluster, control plane nodes should be rebooted one at a time.  We say that in the vSphere updating hardware guide at https://docs.openshift.com/container-platform/4.14/updating/updating_a_cluster/updating-hardware-on-nodes-running-on-vsphere.html.  Contents:

Updating the virtual hardware for control plane nodes on vSphere
To reduce the risk of downtime, it is recommended that control plane nodes be updated serially. This ensures that the Kubernetes API remains available and etcd retains quorum.