Is it a bad idea to keep NetworkManager and network-scripts together on RHEL 8?

I would aim for NetworkManager and update the automation rather than keep the legacy scripts indefinitely. RHEL 9 moves away from network-scripts, so the automation is likely to need attention during an upgrade anyway.

That does not have to mean switching everything at once. Inventory what writes the configuration and what triggers network restarts first, then plan the migration around those dependencies.

For now, focus on which service and automation owns each connection, not just whether both packages are installed. Check what the automation changes and what happens when it runs. A restart that interrupts connectivity is worth treating as a disruptive operation, especially on a remote host.

Test changes with console access available and avoid restarting the whole network service casually. The brief outage alone does not establish that having both installed is the cause.

Keeping the current setup can be a practical middle ground if the automation cannot be changed yet. Just make that a conscious temporary choice and check the automation before an RHEL upgrade. The legacy setup may work on RHEL 8 without being a good long-term dependency.