Looks like Elm got a stealth 0.19 release.
Blog article: http://elm-lang.org/blog/small-assets-without-the-headache
Release notes: https://github.com/elm/compiler/blob/master/upgrade-docs/0.19.md
Installer links: https://guide.elm-lang.org/install.html
Is there any official documentation on what changed with respect to native and event modules?
The summary: native/kernel code and event modules were never intended to be used by package authors; only the core of the language should have them. The blog post from today you linked has a good explanation why: we can do a much better job optimizing if we only allow pure Elm code in packages.
If you were using native code before, I’d recommend you use either ports or custom elements. Would you mind opening separate threads for your specific use cases so that we can help you upgrade?
Here are some threads with more info:
For what it’s worth, this is a public release. It’s not meant to be stealthy at all. 
Finally, a moderation note: I’m going to lock this thread since discussions on this topic have the unfortunate tendency to spiral out of control and we have already debated this past the point of productive return. If you have things you need to upgrade, we’re of course happy to help… let’s just do them in separate threads.