According to JD Walters,
“Doesn't the necessity of slaughter indicate that here indeed we have penal substitution, the infliction upon an innocent animal the punishment which should rightly have befallen the worshipper? Actually, no. In Leviticus 17 we read that what is actually atoning about the whole process is not the killing itself, but the blood of the sacrificial animal: "for the life of every living thing is in the blood. So I myself have assigned it to you on the altar to make atonement for your lives, for the blood makes atonement by means of the life." (Leviticus 17:11) Here we have a very clear statement that the blood is atoning, not because it represents the just punishment suffered for sin, but because it carries the life of that which was offered. What the worshipper is really offering here is life itself.”
In context, we’re not dealing with blood in itself, but shed blood. Because blood represents life, shed blood represents death. Sacrificial blood signifies the death of the sacrificial victim, as in bleeding to death.
“The spotlessness of the sacrifice does NOT however mean that the sacrifice is an 'innocent' substitute which bears the punishment of the sinner. There is no sense in which animals are innocent or guilty, because they do not sin.”
That’s just inept. It fails to draw a rudimentary distinction between a symbol and what it stands for. Penal substitution doesn’t assume that the animal is literally innocent.