Clients supporting this DHCP option (DHCP option 108, RFC 8925) will disable its IPv4 network stack for configured number of seconds and operate in IPv6-only mode.
Clients supporting this option include iOS 15+ and macOS 12.0.1+.
Example command:
set service dhcp-server shared-network-name LAN6 subnet 192.168.64.0/24 ipv6-only-preferred 0