I just used Dear ImGui in a VSCode tab in the last couple of weeks [1] :)

...indirectly by embedding a home computer emulator with a Dear ImGui debugger UI, compiled to WASM+WebGL running in a webview panel, but the idea to use Dear ImGui for tooling UIs in VSCode extensions where HTML+CSS might be too awkward is certainly interesting (think not just "traditional UIs", but also things like noodle-graphs, flame-graphs, diagrams... the vector renderer coming with Dear ImGui is great for that type of stuff).

[1] https://marketplace.visualstudio.com/items?itemName=floooh.v...