As of today's r412 there is support for wrapper extensions with the Linux export option, which the official Steamworks (formerly "for WebView2", but dropped that part of the name now) add-on plugin uses for Linux/Steam Deck support. The code on GitHub for both the wrapper extension sample addon and Steamworks now include the source code changes and build instructions for Linux (a pretty straightforward minimal CMake setup).
We're aiming to get wrapper extension support for macOS in the next release cycle, so at least a couple of months away for that, but then the Steamworks plugin should work on all of Windows, macOS and Linux. After that we'll be looking at the feasibility of dropping support for NW.js entirely, but we'll be asking for community feedback on that at the time, and I suspect we will be supporting NW.js for quite a while longer to ensure nobody with important projects gets left unsupported.