You're right, joevt. The card also works in High Sierra.
I have a 2010 Mac Pro with the GC-Titan Ridge AIC, Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 adapter, and an Apple Thunderbolt Ethernet adapter working on High Sierra (10.13.6). I needed to jumper pins 3 & 5 on THB_C header cable, boot into Windows 10 with the AIC driver installed, and then reboot into macOS High Sierra for it to work.
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Thank you for the update, Factionrider. I wonder if we need to connect the USB 2.0 header cable on the AIC to make the Apple Thunderbolt Display work.
From my knowledge, I might be wrong here: I don’t believe available builds of High Sierra will work with titan ridge. The 2018 MacBook Pro was the first to use the controller and was released with a unique build, if memory recalls.