proposal-private-methods VS proposal-private-declarations

But with the new ECMAScript class fields proposal we can do this easily and gracefully by using private fields as seen below:

Private instance methods and accessors Class Public Instance Fields & Private Instance Fields Static class fields and private static methods

Don't forget that abomination of syntax which is https://github.com/tc39/proposal-private-methods

This is not currently true. But there are proposals, currently in stage 3 for allowing this.

Private instance methods and accessors

There is a proposal for private methods but its not yet standard. However, they are currently supported in chromium browsers and in firefox after enabling a flag. These, however, only work using the class syntax, not in constructor functions like that used in your example.

There one or two additional addon specs like Private Declarations, but the other private specs are not dependent on this. I believe, and I haven't been keeping up with them as much lately (they've been hanging at stage 3 for sooo long), that brand checks was something they wanted to have ready before the rest going out.