Theological Note of the Core Proposition
I am not aware of any theologian who considers the Disciplinary Infallibility of the Church to be de fide in the strict sense. So, technically, what this means is that even if someone were to OUTRIGHT deny the core proposition, the person would not, strictly speaking, be a heretic, a non-Catholic, and outside the Church. Of course, what the theologians probably had in mind were relatively-minor things such as forbidding Holy Communion under both species. To say, however, that the Church could impose (or, according to Gregory XVI, even permit) a Rite of Mass that would be Protestant, "bastardized", offensive to God, and something which cannot in good conscience be offered by Catholic priests or assisted at by the Catholic faithful would be to state that the Church could essentially defect in her public worship and to implicitly reject the Church's indefectibility, so a case could be made for calling that proposition proximate to heresy; but, since it has never been explicitly defined, it still falls short of heresy in the strict sense. This does NOT mean that it would not constitute objectively a grave sin against the Faith to deny the proposition or that the proposition is not certain.