Is the Concept of Universal Salvation a Heresy?

If by “universal salvation” you mean that everyone who ever committed a mortal sin perfectly repents before death and is purified in Purgatory and ultimately arrives in heaven then no, it is not heresy. The Church teaches that all who die in an unrepentant state of mortal sin go to hell for eternity. The Church teaches that hell is real and is eternal but does not teach that any particular person has truly died in a state of unrepentant mortal sin because that is impossible to know.

God predestines no one to go to hell; for this, a willful turning away from God (a mortal sin) is necessary, and persistence in it until the end. In the Eucharistic liturgy and in the daily prayers of her faithful, the Church implores the mercy of God, who does not want “any to perish, but all to come to repentance” (CCC 1037)

If by that phrase you mean that hell is only temporary or that all souls go to heaven regardless of what they did in life and whether they repent then that is indeed heresy. This heresy goes by several names: universal reconciliation, universal salvation, or Christian universalism.