From looking at the MCD there are 4 areas that address RHEL9-specific MCD Logic:
1. NIC Name Persistence (pkg/daemon/daemon.go:2017-2040) - this is currently in progress with https://github.com/openshift/machine-config-operator/pull/5406
2. SSH Key Path Selection (pkg/daemon/update.go:2291) - this determines whether to use new SSH key path (/home/core/.ssh/authorized_keys.d/ignition) vs old path (/home/core/.ssh/authorized_keys). This needs to be updated to include RHEL 10, so that RHEL 10 uses the new ssh key path
3. Realtime Kernel Packages (pkg/daemon/update.go:1756-1758) - this was addressed in this PR https://github.com/openshift/machine-config-operator/commit/6e6477a5393b9ec24a6027f4f1c028d3f4cf48a8
4. Binary Selection for RHEL8 Hosts (pkg/daemon/daemon.go:536-543) - this uses RHEL8-compatible binary when container is RHEL9 but managing RHEL8 host. We will need to handle RHEL10 container managing RHEL8 or RHEL9 hosts.

Half of this work is either complete or has a PR linked to it. We will need to address items [2] and [4] to complete this story.