I think this is the conclusion, but no one has stated it outright. Any video card that has HDMI audio support supports 7.1 audio. So there you have it. If you can play sound via HDMI, it supports 7.1. The rest is configuration between playback software, EDID, receiver capabilities, etc.
Video: JVC RS4500 135" screen in pure black room no light, htpc nvidia 1080ti.
Audio: Anthem mrx720 running 7.1.4, McIntosh MC-303, MC-152, B&W 802d3 LR, B&W HTM1D3 center, B&W 805d3 surround, B&W 702S2 rear, B&W 706s2 x 4 shelf mounted for atmos, 2 sub arrays both infinite baffle: 4x15 fi audio running on behringer ep4000 + 4x12 fi audio running on 2nd ep4000.