cuм Ex, inasmuch as it is merely ecclesiastical legislation, is no longer in effect. The 1917 CIC replaced it, as it replaced any and all laws which came before it (unless and except it says otherwise, or somehow incorporates them into it). So there's not a law governing the Church right now, nor has there been one since 1917, called cuм Ex Apostolotus Officio.
But inasmuch as cuм ex reflects the divine law, it's still in effect, just as the ten commandments are, despite the fact that the Old Covenant is done. The specific penalties that cuм Ex prescribes, then, are not in force (except when they're carried over by the 1917 CIC). But the divine law aspect of it-- mainly, the reiteration that heretics cannot be popes-- is absolutely "in effect," and it would be "in effect" even if there had never been a cuм Ex at all.
Those (like Hesse) who argue "it's not in effect" as though this fact has some effect on the sedevacantist thesis, are simply wrong. And sedes who try to argue that it is in effect don't know what they're talking about either, though they're certainly closer to the truth than those who argue as Hesse does.
ETA: Just because it seems to often be mentioned in the same breath, the fact that excommunicated cardinals are allowed to participate in conclaves (and even be elected, yes, that's true) doesn't bear on the issue at all either.