https://github.com/vyos/vyos-1x/pull/263
https://github.com/vyos/vyatta-cfg-system/pull/121

Actually why do you wan't to disbale IPv6 on the system? I think this is a huge workpackage.

What should happen to all the ipv6 nodes in the system? Should they "vanish" or should they stay as they are?

One example would be disabling IPv6 but enabling either DHCPv6 or configuring an IPv6 address on an interface - thus will result in commit errors as addresses would likely be rejected by the system.

in my opinion it should be always enabled

It's useful when the user is sure he doesn't want IPv6, as it lessens the attack surface, especially if the user doesn't know he needs to configure a IPv6 firewall separately to the IPv4 firewall. Even link-local addresses can be used to launch attacks in the absence of a firewall config.
IMO the configured interface addresses and v6 nodes should become no-ops, possibly print a warning on commit.
On the other hand, leaving IPv6 enabled, would be better to move in the direction of v6 adoption. Personally, I'd prefer this, and leave v6 enabled by default.

it's enabled by default.

Just it handles, if user disable it on live iso before install the system image

Well - making all IPv6 stuff a noop is not coded into VyOS. Can you show real life examples of increased attack surface?

sorry, i'm not a hacker, so i dont know any attach vector. :( But it does not mean that it does not exist.

but in my opinion, if there is an unused stuff in any system, it sould be disabled to reduce the number of possible attacks. thats, we dont use ipv6, so it's disabled system wide.