Yeah, iirc private properties (added in ES2022) are currently the only part of ES6 classes that can neither be created nor accessed using prototype-based code,

Yeah, iirc private properties (added in ES2022) are currently the only part of ES6 classes that can neither be created nor accessed using prototype-based code, to the consternation of some people when they were added. Of course, provate properties can still be readily emulated with a WeakMap in a function scope.