Context. Context. Context. The Council of Trent ended in 1563. It was largely a response to the *first generation* and early *second generation* of Protestantism. No Protestant of the time could possibly plead "invincible ignorance" about Catholicism, the debates were ubiquitous and all-consuming. Millions of Catholics still existed in now-Protestant lands as witnesses to the true Faith, and they were a living sign of contradiction that no Protestant could yet ignore.
Hundreds of years and dozens of generations later, we don't have a living *first generation* of Protestants who made a conscious choice to rebel against the Church. Nearly all adherents of the various Protestant denominations have had family ties to them for many generations. They have developed self-contained theologies that ignore or are not even aware of the Catholic patrimony of their more diatant ancestors. Such people *do* labor, in many, many cases, under the "invincible ignorance" that Pius IX spoke about.
God is not a monster who condemns people for things they have not reasonable expectation of knowing. The Church knows this, and it does not anathematize modern Protestants who are so long cut off from the true Church that they don't realize their error.
I should point out, however, two qualifiers to this. For a catechized Catholic to renounce his or her Faith in favor of anything else (including any variety of Protestantism), and subsequently to die in this state of affairs, is courting spiritual disaster. They are in no different position here than the original "Reformers," all of whom were once Catholic. The other qualifier involves any non-Catholic, today or in any other time, who, knowing the truth of the Catholic Faith, refuses to embrace it.
God *may* cut a lot of slack to non-Catholic Christians (and, by virtue of their baptisms, the Church acknowledges that they ARE Christians, believe it or not!) who labor in His vineyard even while handicapped, but only to the extent that their consciences truly tell them the Catholic Church is false. The Church, which teaches in truth, is the Body of Christ; indeed, the Truth IS Christ, and its fullness cannot be renounced consciously without consequences.
All of this is not just some academic exercise. Unity in faith was quite evidently important to Jesus. On the night He was betrayed, doubtless somewhat preoccupied with facing a heinous death that He already knew the full scope of and dealing with the human fear that engendered, He managed to focus His prayer to His Father on the unity of faith His followers should have. "I do not pray for these only, but also for those who believe in me through their word, that they may all be one; even as thou, Father, art in me and I in thee, that they also may be in us, so that the world may believe that thou hast sent me." (John 17:20-21, but read the whole chapter in context.)