IPv6 doesn't use ARP. It uses Neighbor Discovery (ND), which uses ICMPv6. Proxy-arp was used for IPv4 networks where some devices had a wrong subnet mask. The proxy-arp feature on the router would reply to ARP requests for off-net addresses so that such clients would send their traffic to the router so it would actually act as a default gateway. In IPv6 you are expected to correctly configure your network and devices, which is made easier by sending that information to all attached devices with a Router Advertisement (RA).
There is an IPv6 proxy-nd feature that simulates bridging different networks together where a real bridge isn't possible. Cisco routers don't do proxy-nd (yet and/or by default) so there is no need to disable it.
IPv6 doesn't use ARP. It uses Neighbor Discovery (ND), which uses ICMPv6. Proxy-arp was used for IPv4 networks where some devices had a wrong subnet mask. The proxy-arp feature on the router would reply to ARP requests for off-net addresses so that such clients would send their traffic to the router so it would actually act as a default gateway. In IPv6 you are expected to correctly configure your network and devices, which is made easier by sending that information to all attached devices with a Router Advertisement (RA).
There is an IPv6 proxy-nd feature that simulates bridging different networks together where a real bridge isn't possible. Cisco routers don't do proxy-nd (yet and/or by default) so there is no need to disable it.