Since this issue has been verified on OCP 4.10, marked this bug as VERIFIED, thanks.

We can't close 4.9 or 4.8 bugs as they are verified in 4.10.  There could be chance these fixes would have introduced regression in older releases. Can you pls test them in 4.8 and 4.9 separately. 
Also testing upgrade from 4.8 to 4.9 is one important use case.  Because while upgrade we will have old secret which we will have to use old method to decrypt the username to deconfigure  windows node.

This bug has been verified with upgrading OCP 4.8 to 4.9 and passed, thanks.

Steps:
Before upgrading, BYOH username encrypted as:

# oc get node ip-10-0-151-53.us-east-2.compute.internal -oyaml
apiVersion: v1
kind: Node
metadata:
  annotations:
    k8s.ovn.org/hybrid-overlay-distributed-router-gateway-mac: 00-15-5D-C0-C2-DF
    k8s.ovn.org/hybrid-overlay-node-subnet: 10.132.2.0/24
    machine.openshift.io/machine: openshift-machine-api/sgao-aws-7qpjh-windows-byoh-us-east-2a-mr2ln
    volumes.kubernetes.io/controller-managed-attach-detach: "true"
    windowsmachineconfig.openshift.io/pub-key-hash: 1df2c166b1c401180523270e9cf6bc2cd2724b9279ea65668a3b95298525a0f5
    windowsmachineconfig.openshift.io/username: '-----BEGIN ENCRYPTED DATA-----<wmcoMarker><wmcoMarker>wx4EBwMIrYn4SHVJyARgyS7HTkOLhHcyphTvA/f/32vS5gGTXazWAqvzzWfRvWpT<wmcoMarker>JHYGf0N7g+z46k3Gzdqw/s7iRA3vQbFAGOq/f59k2qEu/legD/zbn5n52BIheMES<wmcoMarker>zmLgRQA=<wmcoMarker>=DEKe<wmcoMarker>-----END

After upgrading, it changed to:

# oc get node ip-10-0-151-53.us-east-2.compute.internal -oyaml
apiVersion: v1
kind: Node
metadata:
  annotations:
    k8s.ovn.org/hybrid-overlay-distributed-router-gateway-mac: 00-15-5D-90-15-0B
    k8s.ovn.org/hybrid-overlay-node-subnet: 10.132.2.0/24
    machine.openshift.io/machine: openshift-machine-api/sgao-aws-7qpjh-windows-byoh-us-east-2a-mr2ln
    volumes.kubernetes.io/controller-managed-attach-detach: "true"
    windowsmachineconfig.openshift.io/pub-key-hash: 1df2c166b1c401180523270e9cf6bc2cd2724b9279ea65668a3b95298525a0f5
    windowsmachineconfig.openshift.io/username: wx4EBwMIJuGDh7tNY89gqxopFlNt0mR93lLCmyOFgtPS5gHvhEyjxebORn5cZgHV<wmcoMarker>IwWX8shN3KZnCcoTcXLm3Gio/gt8LCxuoBCHI+lyQ4h7qEFPvgVEQs1FN+p8PTOT<wmcoMarker>GgjgswA=<wmcoMarker>=tbGW
    windowsmachineconfig.openshift.io/version: 4.0.1+f66f098

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (Windows Container Support for Red Hat OpenShift 4.0.1 product release), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2021:4757