IMO, the one thing that made JS an absolutely horrid language is that it failed to do what every other programming - or scripting! - language out there did: deprecate the ugly/bad shite.
This could have been trivially handled, even within web browsers. Simply adding a version number to a script tag could have told the web browser to target only that version. The added payload needed to be included with the web browsers would have been tiny, especially if a diff system was implemented that allowed a specific runtime to be auto-assembled and spun up for every version needed.
