Certainly considered heresy from the days of the early Church to present; Protestant and Catholic, at the council of Nicea (Arius), and it persists today in Islam, JWs, Mormons. Certainly mentioned in the Lutheran Confessions; Arians and Anti-trinitarians are mentioned separately in the Formula of Concord:
Error of the New Arians.
28] That Christ is not true, essential, natural God, of one eternal, divine essence with God the Father and the Holy Ghost, but is only adorned with divine majesty inferior to and alongside of God the Father [is so adorned with divine majesty, with the Father, that He is inferior to the Father].
Error of the Anti-Trinitarians.
29] This is an entirely new sect, not heard of before in Christendom, [composed of those] who believe, teach, and confess that there is not one only, eternal, divine essence of the Father Son, and Holy Ghost, but as God the Father, Son, and Holy Ghost are three distinct persons, so each person has its essence distinct and separate from the other persons of the Godhead; and that nevertheless they are either [some think] all three of equal power, wisdom, majesty, and glory, just as otherwise three men are distinct and separate from one another in their essence, or [others think that these three persons and essences are] unequal with one another in essence and properties, so that the Father alone is properly and truly God.
30] These and similar articles, one and all, with whatever other errors depend upon and follow from them, we reject and condemn as wrong, false, heretical, contrary to the Word of God, the three Creeds, the Augsburg Confession and Apology, the Smalcald Articles, and Luther's Catechisms, against which all godly Christians of both high and low station are to be on their guard as they love the welfare and salvation of their souls.
31] That this is the doctrine, faith, and confession of us all, for which we will answer at the last day before the just Judge, our Lord Jesus Christ, and will neither secretly nor publicly speak or write anything against it, but that we intend by the grace of God to persevere therein, we have after mature deliberation testified, in the true fear of God and invocation of His name, by signing with our own hands [this Epitome].