It seems like the link for "Backing up and restoring the undercloud and control plane nodes" mentioned in the below documentation is wrong.

 Chapter 1. Preparing for a minor update

Considerations before you update your RHOSP environment

To help guide you during the update process, consider the following information:

• Red Hat recommends backing up the undercloud and overcloud control planes. For more information about backing up nodes, see Backing up and restoring the undercloud and control plane nodes. <============================ (*)here

The link should be the below.

 https://docs.redhat.com/en/documentation/red_hat_openstack_platform/17.1/html/backing_up_and_restoring_the_undercloud_and_control_plane_nodes/index