Just adding my 2 cent here.
I got the Titan ridge (Rev 1.0) connected to Gigabyte x570 Aorus Xtreme (Rev 1.0). I’ve connected pins 3 and 5 on the card. I’m running Manjaro Linux on my desktop box
Doing that allowed me to use my two displayport monitors via a thunderbolt 3 hub (CalDigit USB-C Pro) but not USB. The hub and card didn’t show up in boltctl (but did in lscpi) until I connected the USB header. Now everything works (although I haven’t tested USB throughput but I presume the USB is used to configure the card).
Plug and play seems to work fine. I can swap my monitors and peripherals via the hub with my MacBook Pro and back and everything seems happy.