In the 1930s and early 1940s Fr. Leonard Feeney (1897-1978) was known to the public mainly as a writer of better-than-average poetry and of popular books such as "Fish on Friday." From the late 1940s until his death he was known instead for his rigorist interpretation of the maxim "extra ecclesiam nulla salus" ("no salvation outside the Church"). Adherents to his interpretation became known as "Feeneyites."
Ordered to stop teaching his interpretation, Feeney refused and was excommunicated, not technically for teaching heresy but for disobedience. He was reconciled to the Church before his death, and the excommunication was lifted. Some of his followers have tried to construe the reconciliation as a Vatican affirmation of Feeney's theology, but, since the excommunication did not extend beyond a matter of obedience, the lifting of it did not extend any further.
Catholic Answers
His teaching was not heretical. His refusal to "keep it quiet" was the reason for his excommunication.
You seem to say: Feeney was not excommunicated for heresy, but for disobedience, THEREFORE his teaching was not heretical.
This does not follow. There are plenty of heretics running around who have never been excommunicated for ANYTHING.
Feeneyism is a heresy, and a particularly harmful one. It stems from, and fosters, neurotic anger and fear. It scandalizes people and drives them away from the Catholic Church, because it is contrary to common sense and the gospel.