I hadn't taken any interest in the topic of Jansenism, and only looked this up because it mentioned Blaise Pascal, who was a very effective Christian.
It think the following paragraph encapsulates the reason it was condemned as a heresy, in that it denies "the role of free will in the acceptance and use of grace." I suppose it was seen as a form of Calvinism trying to sneak in via the Catholic Church's back door during the turmoil of the post-Reformation years.
Jansenism - Wikipedia
Pascal had an attraction towards Jansenism at one stage (in his younger years) starting in 1646 when he was 23, but he turned away from it a couple of years later, followed by what others have termed his "secular period".
Just a few years after that, in 1654, he had his notable religious experience as recorded on a brief note sewn into his coat.
On the 23 of November, 1654, between 10:30 and 12:30 at night, Pascal had an intense religious experience and immediately wrote a brief note to himself which began:
"Fire. God of Abraham, God of Isaac, God of Jacob, not of the philosophers and the scholars..."
What makes this notable is the fact Pascal was himself very much into the philosophy of religion.
He regained his religious enthusiasm after this event. However he always suffered from ill health, and died in 1662, at the age of 39.
I think I read somewhere that Malcolm Muggeridge, the acerbic journalist who in his early days had dabbled with Communism, then became protestant Christian, and finally Catholic (influenced by Mother Teresa whom he helped introduce to the West), wrote somewhere that he thought Blaise Pascal lived one of the most victorious Christian lives of all time, despite his chronic ill health.
But I'm relying on my memory there from an article I read quite some time ago.
PS - Yes, I think it would be regarded as "heretical" today because it denies the role of our free will in accepting or rejecting God's grace. We have a choice.