Access Steam features in the Windows WebView2, macOS WKWebView, and Linux CEF exporters. Requires r407.2+ for Windows, r417+ for macOS, and r412+ for Linux.
You write "However the in-game overlay is not currently supported on Windows, macOS or Linux systems other than the Steam Deck." which means this addon will not allow a game to pass steam's validation process. Valve employees will flag the game as non functional if they cant initiate the overlay. Therefore no one is going to have a Construct 3 steam game once NW export becomes unsupported.
I have never seen anything saying the Steam in-game overlay is required to publish to Steam. Do you have a reference for that?