Since part of the current language specification is that parallel is allowed after any operator, we cannot simply remove it. We could add warnings to tell users when parallel isn't doing anything for them, and deprecate it's usage for those operators.
But even then I'm not sure how useful this is. Some operators will sometimes make use of parallel and sometimes not, depending on implementation (e.g. join would use it for hash join, but not for merge join). So users will always have to know when parallel is and isn't helping them.