Namely they voted for encapsulation / hard-private [#33] as well as comparing private properties of two classes [2], which as a result makes the only possible syntax the one with the `#`.
Unfortunately this comes at the price that once browsers implement this it will become history ( same as the `with` statements [3] and `labels` with `continue` [4] ). Because of backwards compatibility these will never be removed.
Maybe in 2022 TC39 will decide that "use static-type;" will be a mode, such as strict mode, where you can write statically typed JS ( great idea, btw ), where this proposal will make no sense. Another "use class-inheritance;" mode would also finally end the abuse of prototype inheritance with syntax sugar. In this future the `#` will be just a huge annoyance.
#33 : https://github.com/tc39/proposal-private-fields/issues/33
2 : https://github.com/tc39/proposal-class-fields/blob/master/PR...
3 : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...
4 : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...