I thought minifiers were supposed to keep licenses (they can detect the license header). I suppose this could get turned off or messed up by accident with the gigantic custom webpack bundlers even stuff like create-react-app ship with.

Lots of libraries don't put the license in the header, only in a separate LICENSE file.

I've looked at Webpack's output before, and I don't recall seeing any headers there.

Webpack definitely keeps the license headers even when minification is on. They have to start with something like "@license".