There doesn’t seem to be a good way for a simple network-connected snap to check if ipv6 is enabled. Normally, you can just check the contents of /sys/module/ipv6/parameters/disable, but access to this is only granted by the kernel-module-observe interface, which seems overkill.
Does it make sense to add read access to /sys/module/ipv6/parameters/disable to the policy for the network interface?