Daegus,
Objectively considered, Protestantism is a heresy of course. It involves a repudiation of Catholic truth. But this statement may not necessarily be true, as it pertains to their interior disposition - they may be inculpably ignorant, and so in good faith. If they are ignorant through no fault of their own and are willing to obey the will of God and the truth He has revealed, as best as they know it, then this would not be true.
For the vast majority of people it is true. How many Protestants above the age of reason do you know that don't know what the Catholic Church is or have never heard of it in their lives? How many of them would convert to the Catholic faith if you could prove it was true beyond question? I doubt the number is very high. You also need to consider that external actions are more often than not reflective of the interior disposition of the soul.
On another note, you should be very careful with how you deal with the topic of "invincible ignorance" and the like. Inculpable/Invincible ignorance has never been historically used to apply to Protestants, and it really has only ever applied to those who the gospel has not yet reached. When speaking of those people, they will not be condemned for the sin of unbelief, but will be condemned for other mortal sins. If these people were truly of good will, God would reveal His truth to them in one way or another. Besides, if these people were really invincibly ignorant, they wouldn't actually be Protestants at all, but Catholics who are erring in the faith.
Once a Protestant makes their profession of faith (which virtually of them do or will do at some point), that is when they cease to be members of the Church and make the decision to adhere to a false sect and thus become heretics. If they were truly of good will and "inculpably" ignorant, God would find a way to reveal his truth to these people. Another thing to consider is that the vast majority of Protestants in the world are in places where the presence of the Catholic Church is hardly invisible. The Catholic faith is definitely not foreign to Europe (as it built Europe..) or the North Americas. It is not foreign to South America (for the same reason as Europe), it is not foreign to Australia and it is not entirely foreign to Africa as Catholic missionaries have been proselytizing long before Protestants came about. It is only more or less foreign to the majority of Asia, as Christianity has heavily been suppressed in Asia (this is especially true for China).
In summary, I have no reason to believe that there's anything other than a
very small amount of people who are "inculpably" ignorant as you say. Protestantism's existence has its basis in its rejection of the Catholic Church. There is not much of a way to get around that.
Like spouse said, unless one lives in a remote jungle (which Christian ideals would probably not even reach to begin with), it's highly unlikely that such an ignorance happens to very many people, if anyone at all.