Amy reached out out of band, so I am hoping to help answer the question.
There are several reasons why disabling IPv6 on WAN interfaces is not supported, but I'll focus on the main ones. Firstly, our cloud-connected products are designed to provide management flexibility using IPv4, IPv6, or both, ensuring they are future-proof. Secondly, similar to IPv4, we maintain this approach for Dual Stack to avoid unnecessary complexity, as disabling IPv6 is not a widely requested feature.
Additionally, when IPv6 is enabled on the MX WAN uplink, it is limited to MX communications and does not impact client traffic or behavior. This is particularly true if the LAN/VLANs are not configured for IPv6, as no IPv6 prefix is available or expected to be delegated in this setup. If IPv6 is not desired at all, it can often be disabled on the upstream device itself too.