Migrating from webpack 2 to 3, should involve no effort beyond running the upgrade commands in your terminal. We marked this as a Major change because of internal breaking changes that could affect some plugins. So far we’ve seen 98% of users upgrade with no breaking functionality at all
Again, I really appreciate all the work done to make this smooth, but there's still a little underlying "compatibility matters less than features" sentiment here.
It didn't say 'plugin interface'. It said 'internal changes' that could affect some plugins. The way I read that is some plugins could be relying on internals (that they technically shouldn't, they may have good reason ofc).