when writing them down, we should definitely differentiate for each invariant which:

• are true at the moment
• were intentionally held vs accidentally held
• has committee consensus to be held moving forward

var iframe = document.createElement('iframe');
iframe.src = location.href;
document.body.appendChild(iframe)

var final = iframe.contentWindow.eval('(onThen, onCatch) => { window.data = {onThen, onCatch}; }')

var thenFinally, catchFinally;
Promise.prototype.finally.call({
  then(thenFinally_, catchFinally_) {
    thenFinally = thenFinally_;
    catchFinally = catchFinally_;
  }
}, final);

console.log('thenFinally', thenFinally.constructor === Function);
console.log('catchFinally', catchFinally.constructor === Function);

Promise.resolve({
  then: final
});
Promise.resolve().then(() => {
  console.log('onThen', iframe.contentWindow.data.onThen.constructor === Function);
  console.log('onCatch', iframe.contentWindow.data.onCatch.constructor === Function);
});

One thing I didn't get into is how having an explicit intent for an ObjectPlaceholder being an exit from the immutable structure is that it allows predicates that compare the immutable shape of a record/tuple oblivious of the exits, without running into the collision issues that strings have (and for which unique symbols were created in the first place):

const eventHandlerTag = Symbol('EventHandler');
const domElementTag = Symbol('Element');
const rec1 = #{type: eventHandlerTag, ref: ObjectPlaceholder(() => {})};
const rec2 = #{type: eventHandlerTag, ref: ObjectPlaceholder(() => {})};
const rec3 = #{type: domElementTag, ref: ObjectPlaceholder(document)};
assert(rec1 !== rec2);
assert(sameShape(rec1, rec2) === true);
assert(sameShape(rec1, rec3) === false);

We also could consider introducing something like a debugger.meta meta-property to facilitate debuggability, i.e.:

// assumes `debugger.meta` is ECMA262-defined, but a debugger can add their own methods to it

const placeholders = new WeakMap();
export function box(value) {
  const sym = Symbol();
  placeholders.set(sym, value);
  return sym;
}
debugger.meta?.addCustomDescriptionGenerator((value, describe) => {
  if (typeof value === "symbol" && placeholders) {
    return `Placeholder (${describe(placeholders.get(value))})`;
  }
});