Went through this exact process last week. Did they really need to remove CommonsChunkPlugin? Feels like breaking stuff for a slightly nicer API. If this were React we would've had a deprecation warning for a few versions.
If this were React, you would have got docs with a release.
Choosing to release before documentation is done, for a tool developers are supposed to use as a black box, which has a lot of strange configuration that is required, is a really strange choice for something with so many breaking changes.
[0] https://www.npmjs.com/package/webpack