I'd argue multi-window would require a substantial redesign of the graphics layer, so much that the maintainers would be uninterested in supporting it, and consider it out of scope.

I can imagine IME support being integrated, but a giant pain, due to the wide variety of APIs across platforms, the fundamental disconnect in how text input is done in games vs. elsewhere, not to mention the "Linux wars": will you support ibus or fcitx?

So it requires someone to step up and do the work, and don't be fooled: it's a lot of work.

Godot 4.0 recently acquired multiple window support for the editor (which is built with Godot's UI and rendering system) and APIs for applying it to your own games

Just to clarify for those following along. Godot IDE is using the same UI components that you can use in your games. Godot 4 will support multiple windows. They're working on RTL I believe and it actually has a very robust code editor with debugger in the editor. Most of the points being raised here don't seem to be based on actual experience with the framework.