I know the question is old, but come on!
IPv6 is nothing more than IPv4 with an amount of 128 bits, in short.
It includes letters, numbers and addresses our current demand for IP addresses.
A server ONLY under IPv6 may decrease a DDoS attack, since only IPv6 will access it, but it does not mean that it is immune.
I manage a hosting server that hosts a few large sites and have already received DDoS attacks that included both IPv4 and IPv6. Having one or the other disabled will only decrease the range of IPs that would access your server.
And I do not recommend that you deactivate IPv4, especially in Brazil, many operators have not yet adhered to IPv6 in all states or have upgraded their routers, most of which are still IPv4.