Yes, in summary, NW.js is fully removed in r450+. We marked it deprecated in r421 back in January, and support for NW.js will continue in the r449 LTS releases until the end of 2026, so that's an approximately two year time span over which it will have been phased out.
This also came up on Reddit and I explained in much more detail why we've done this in this comment. I won't repeat what I wrote there here, so do go and read that for the background on this.
Broadly speaking it is time to move everyone over to our own desktop export options. The maintenance work is a key consideration as we are a small company with limited resources, and maintaining two options for desktop exports is a significant drain on resources, particularly given how tricky NW.js is to maintain and customize.
runtime.js:1 Uncaught (in promise) TypeError: this._runtime.IsNWjs is not a function
Oops, that shouldn't happen - fixed that for the next beta.
I also presumed that we could still choose to use NWJS, but no support provided from Scirra
If you are happy with that, you will still be able to use NW.js in the r449 LTS releases indefinitely, even past their official support period. We have never taken down old releases. Every past release we've ever made is still available on the releases page and it'll stay that way.
What if Scirra decide WebView2 is not the path to go with
I am well aware how difficult these transitions can be, and I can assure you we absolutely plan to stick with WebView2 (and our other desktop export options) permanently, and would only change technology again in extraordinary circumstances. In particular with Windows WebView2 and macOS WKWebView we are using the webview built in to the system, which is a key thing we want to avoid having to ship an entire browser engine with exported projects. No other technology can offer that other than the system built-in option.
I'd have thought encouragement of Node.JS would be the path to promote
It was different in the past, but at this point, the entire node.js component of NW.js is basically redundant. It provides a large API surface, but geared towards server-side development. It does not do things like out-of-the-box support for integrating the Steamworks SDK, and in fact it makes that a lot harder, as all your C++ integration has to go through the node.js layer, which also has some really awful compatibility issues (at least in NW.js). Our new wrapper extension model basically means you can write C++ code that directly interacts with JavaScript via a simple messaging system, which makes it much easier to integrate custom native code.
I do apologize for the inconvenience of these technology switches, but in the long term they are absolutely necessary. I know they are painful and these are not decisions we take lightly. As I mentioned in the r450 release notes, Construct 3 has been around for about 8 years now, and we need to think about where we'll be in another 8 years and longer. These kinds of decisions are difficult but important to ultimately ensure Construct is still a great product that keeps improving and remains competitive.