I think 11ac support for WPA3 is a must. With good WLAN deployments there is no need to switch APs from 11ac or even 11n to 11ax for at least couple of years, before most of the STAs will be 11ax-capable so you will be able to benefit anything from that standard.
Other vendors have announced WPA3 support with their 11ax APs, so I expect Extreme would be the same. The real question is whether WPA3 support will be backported to any 11ac APs. Again, I see other vendors are promising this so it's possible at least.
My understanding at this early stage is that the specification is not fully defined, essentially WiFi Alliance have merely announced plans for WPA3.
The meat and bones of the specification have yet to be defined.
You should stay in touch with your local SE who should be able to advise you of more detailed timeline and hardware/software support later in the year.