I thought about that too, but this breaks down once you realize that setting obj.key = undefined; will mutate an object into something which is neither "the key exists and its value is null" nor "there is no such key". The difference can e.g. bite you in things that iterate over the keys of an object or function that do that internally (like deepEquals implementations).

I guess in TS2 there would now also be a difference between a field defined as key: string | null | undefined and a field defined as key?: string | null | undefined, because the first one requires setting the key to one of the values on initialization and the other one not.