The Catechism on the Council of Trent on the fruits of Christ’s passion:

Note that Christ’s death was propitiatory, appeasing. He made satisfaction for our sin. It was not a legal exchange. The nominalistic atonement theory of strict penal substitution is a heresy and empties Good Friday of its real importance.

Too many Christians view the Cross as the moment when Christ literally took on the exact punishment man deserved for his sin or see it as a breach in the Holy Trinity in which the Father pours out His anger on the Son. Propitiation is the key. Christ soothed the Father’s wrath...

...against us. He did not endure it.

Aquinas:

Propitiation, again, is a soothing of anger, not a redirection of it. In the Passion, Christ offers to the Father a gift more pleasing than our sins were displeasing. It is a true reparation or satisfaction. He did not take on the punishment of eternal separation from God...

...as some latter day Christian heretics conceive of it. Nor would such a punishment even be metaphysically possible.

Penal substitution, as such, is only orthodox then when qualified or by way of analogy. It becomes heretical when taken to the legal and objective extreme.