Generally today “heretic” is given the more precise definition that includes culpability. In past times, it was used in a more broader sense to include anyone who publicly professed a heresy or was a member of a heretical community, whether in good faith or not–that approach was merely concerned with what was external, without trying to read hearts.
Nowadays, there is a general presumption that those born into such separated communities are not guilty of the sin of separation (heresy or schism). Of course, it is merely a charitable presumption and no doubt varies from Protestant to Protestant. If someone remains a Protestant due to disregard for the truth, they would be guilty of the sin of separation. If they are making a good faith effort to learn the truth and observe all Jesus taught, then they are not.
The definitions of Trent use “let us be anathema” like St. Paul does in Galatians 1:8 to say the preceding clause (“if anyone says X”) is a different Gospel. It does not intend to levy a definitive judgment on particular souls who say those things (some are guilty of heresy some are not, as discussed above).
Of course, Protestants are not completely separated from the Church–the Baptismal bond always remains. Here’s how Ott’s Fundamentals of Catholic Dogma explained it (this was before Vatican II–that Council said nothing new in this regard):
Although public apostates and heretics, schismatics and excommunicati vitandi are outside the legal organisation of the Church, still their relationship to the Church is essentially different from that of the unbaptised. As the baptismal character which effects incorporation in the Church is indestructible, the baptised person, in spite of his ceasing to be a member of the Church, cannot cut himself off so completely from the Church, that every bond with the Church is dissolved.
(note, this is using “heretics” in the older, broader sense and “member” in the sense strictly defined to mean dogmatic unity/full unity in faith and hierarchical unity–what we might now refer to as “full incorporation”).
Of course, Jesus did not tell us to judge hearts, but to teach everyone to observe all He taught–and since Protestants by definition do not observe it all, they are included in the Great Commission just like everyone else. “Ecumenism” is just the term used to describe efforts at corporate reunion as distinct, but complementary effort to reconciling individuals.
