Is AC wifi on ASUS RT-AC56 supported?

Is the 802.11AC wifi supported (it is not in OpenWRT)?

According to it's BCM4XXX, so unfortunately not.

You can install AsusWRT-Merlin, but you'll be stuck on the old Kernel that the binary ko is compiled for.

Generally speaking, your safest option for AC wireless devices (especially wave 2) is anything with the ath10k.