proposal-private-methods VS proposal-private-fields-in-in

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.

I believe this was true, but it is becoming increasingly untrue as the spec gives more and more abilities and semantics to classes. Like, as you say, private fields, and more on the way.

Ergonomic brand checks for Private Fields

Ergonomic Brand Checks for Private Fields: Check if an object has a private field, and if not, have some fallback behavior.