The term "immediate mode" is only about the API, not about how things are implemented under the hood, e.g. an immediate mode UI can map to traditional stateful UIs, it's just not done yet very often because of ImGui's roots in the gamedev world (where it is important to easily integrate with existing game rendering engines).
My comment was written from seeing that experiment but also implemented an experiment for a React style declarative API over Win32 earlier and realizing how bad developer ergonomics became together with realizing doing it how close it is to an imgui flow.
Still the fact is that this is the only one I know of and in general accessibility hasn't been a big concern so far (And the imgui name isn't an accident considering the history).
Anyhow considering how laborious "modern" API's like UWP is, how badly declarative API's continue to map to C++ and the frequency at which gamedev rooted developers are asking for how to do "native UI"'s in C++ the time is definitely ripe for a serious one to emerge, the question is though if there will be one with enough momentum to become truly useful or if we'll end up with a bunch of small hacks that kinda covers some bases but don't end up being a good option for the majority.
https://games.greggman.com/game/imhui-first-thoughts/
The term "immediate mode" is only about the API, not about how things are implemented under the hood, e.g. an immediate mode UI can map to traditional stateful UIs, it's just not done yet very often because of ImGui's roots in the gamedev world (where it is important to easily integrate with existing game rendering engines).