That the doctrine was not followed by nuns prior to Vatican II is highly unlikely. That “not every person or every organization upholds church teachings” is of course true, and especially true that in particular British, Russian and American press are rabidly anti-Catholic. This “dumping babies into a septic tank” thing has every mark of being an anti-Catholic media hoax.
... except that there are skeletal remains in a septic tank recorded on plat maps of the former property belonging to the home, and there are 796 deaths of children from that home for which there is no record of burial, annalex. The bulk of the reporting is being done by Irish press. The people who pushed for the government inquiry that is now ongoing are Irish Catholics. Fobbing off every problem that crops up on some anti-Catholic conspiracy is not rational thinking. They’re human and therefore fallen and sinners as are we all. They are capable of error. To my knowledge there was no pre-Vatican II doctrine of infallibility regarding the behavior and moral judgment of nuns running homes for unwed mothers.