I don't see much use in that proposal. Function memoisation is a niche. It can be trivially implemented on top of a Map.
What would be real benefit to all ECMAScript developers would be advancing on the Map/Set features, like GitHub - tc39/proposal-policy-map-set: A TC39 proposal for Maps and Sets with cache replacement policies like LRU and LFU. or Custom hash and equal functions for Map and Set collections e.g. in the collection normalisation proposal or a simple WeakRefMap.