If it is a semver-major change, I'll throw in the changes to socket_error too. I was hoping it wouldn't be breaking, but looking at it now it probably is.

extensions and protocol were both already in the opening channel, but now when not present, they will be an empty string rather than null. It does also change the order of when the opening event is fired, but this isn't as important, but still potentially noticeable if you had something along the lines of:

const events = []
const ws = new WebSocket('ws://localhost')

diagnosticsChannel.subscribe('undici:websocket:open', () => events.push('dc open'))
ws.addEventListener('open', () => events.push('ws open'))

console.log(events) 
// before: ['dc open', 'ws open']
// after: ['ws open', 'dc open']

I'd say we ship and revert if we broke anyone.