Are the display outputs for the Ryzen ITXs Freesync compliant?

I ask because I’m interested in Ryzen 5 2400G.

Has to be Displayport 1.2 or better.
The Asus Prime X370-Pro has a Displayport 1.2 on it, so in theory it should. Depends if the onchip GPU supports it.

yes but only on displayport

I thought Freesync was working on HDMI as well now…

Depends. Requires RX Vega, HDMI 2.1b (or so, too many versions), HDMI VRR and FreeSync 2.0