Deleting the methods is needed for #22567 to pave the way to make it easier to implicitly call the sync_all
member function.
Without the methods being deleted, nothing prevents developers from adding calls to it. As history showed, developers will add calls to it. For example, see commit eb02dbba3cd9f7294cd81e268cf85a1de7a71d02 from today or the first commit in this pull request.