The formal support document shared by Apple does not mention that iMac Pro whether or not supports Pro Display XDR, but according to experiments, Pro Display XDR is indeed supporting iMac Pro.

In the Pro Display XDR tech specs, Apple listed Macs that able to take advantage of the immersive 6K display, although you need to make sure that it must be running on macOS Catalina 10.15.2 or later:
Now the question is will Display XDR support iPad Pro?
Image Via Thomas Grove Carter

In the Pro Display XDR tech specs, Apple listed Macs that able to take advantage of the immersive 6K display, although you need to make sure that it must be running on macOS Catalina 10.15.2 or later:
- Mac Pro (2019) with MPX Module GPUs
- 15-inch MacBook Pro (2018 or later)
- 16-inch MacBook Pro (2019)
- 21.5-inch iMac (2019)
- 27-inch iMac (2019)
- Any Mac model with Thunderbolt 3 ports paired with Blackmagic eGPU or Blackmagic eGPU Pro
Now the question is will Display XDR support iPad Pro?
Image Via Thomas Grove Carter