Disabling IPV6 firewall with IPV6 disabled

The risk is that once you add a device that uses IPv6, you won’t have protection. Leaving the protection on covers you when you make changes to the network, and you don’t have to remember that you turned a protection off.

If the router simply won’t process IPv6 packets, then that’s a layer of protection, until you turn it on to allow an IPv6 device.

How much of a risk that might be to you is something only you can determine.