Remember that the Webpack team has shown to be actively hostile to people who just want to keep their 2-year-old codebases work without doing needless upgrade work. They removed the webpack 1 docs [1], so if you have a few-years-old codebase that you want to get up and running, and somehow you get a webpack error, the message from Webpack is "screw you, upgrade to webpack 3 first and only then we'll allow you to read the docs".
I have no reason to not expect the same to happen with Webpack 4, and the inevitable Webpack 5, 7 months from now. Webpack 2/3 docs will be subtly removed, retired and forgotten. We're all forced to keep doing work to make something work that already worked perfectly fine.
Now, I'm well aware that an angry rant on the internet about the voluntary work from open source contributors is not particularly constructive. I've singled out the Webpack team here, but the reality is that a big part of the web dev ecosystem, across languages, does not care much about backward compatibility. I'd love for that attitude to change.
In semver, every major version is a tragedy.