You probably need to think from a manufacturer’s point of view. If that electronics costs $10 per unit and has to go into 500,000 production units, the bottom line is affected by $5 million. And how many customers will use it? “Ah, no one else offers it, let’s skip it.”

Yamaha told me that the HDMI standards organization's requirements for HDMI audio extraction requires that there also be a discrete HDMI output for zone 2.

Mirrored HDMI outputs (Denon AVRX2600 and down) = no HDMI audio to zone 2
Discrete Zone 2 HDMI output (Denon AVRX3600 and up) = HDMI audio to zone 2

Ah, interesting.