Attempted to call an undefined method named "newInstance" of class "Swift_Message"
Actually while posting the question and linking to the documentation, I was surprised to see it was updated
Now it is
$message = (new \Swift_Message('Hello Email'))
instead of
$message = \Swift_Message::newInstance()
->setSubject('Hello Email')
since the release of swiftmailer6 according to the changelog
https://github.com/swiftmailer/swiftmailer/blob/master/CHANGES#L24
it's a pity there wasn't a "deprecated" period.