1. Install and start Reallusion Kinect One Plugin
2. Check there motion capturing works.
3. Start Iclone
4. Click on CC3 Character
5. Click on Modify panel
6. Observe there is no "Device mocap" button

Expected result:

"Device mocap" button should be present in IClone to be able to use bought Reallusion Kinect One plugin.

Ah. I've found it now.. what I've feared.
https://kb.reallusion.com/Product/53046/Is-the-Kinect-Mocap-compatible-with-iClone-8

You discontinued support for plugin, you've then wanted to users to buy separately and now you just decide to scrap it altogether.
I don't remember promotion for IClone8 mentioning it will be a downgrade in some sense. :)

Ok, I will remember this for any other purchases from Reallusion in the future.

There is no reason to drop support for Kinect plugin except being lazy to maintain it in GUI.

This is outrageous. This policy to sell everything as a bonus without dedication to it's own products or users.

I can open it in Iclone7 but I guess it wouldn't open my IClone8 project file?

I wouldn't be even surprised now if I lost my licence for IClone7 because of the upgrade :D So probably if I want IClone7 I need to buy it again, isn't it? ,-)