Within game development it is predominantly immediate mode as it more closely resembles the main game loop. With other applications I'm not sure, some I know just use ImGui and run an immediate mode UI, others would be retained mode.

In my case I'm looking for a C++ UI library, not .Net. Also not a fan of immediate mode GUIs either, so would want something more traditional.

I don't want to write my own, so will probably try Qt again, but might switch to a custom library in time.