God's nature is eternal, so the way to act in accordance with His nature is therefore also eternal and does not change over time. For example, if it changed over time that at some point in the future it became righteous to commit adultery, then God's righteousness would not be eternal, and the same is true of God's holiness.
Eating unclean animals has always been an abomination against God's holiness, so that has not and will never change, though it is possible that there is something that caused certain animals to become unclean through the Fall. Unclean animals tend to be predators and scavengers, and animals did not exist in that capacity before the Fall, but after death was introduced, it predators and scavengers became necessary to clean up the environment, and this increase of toxins is a large part of why there are night and day differences between how healthy it is to eat clean or unclean animals. There are time laps videos on Youtube of oysters cleaning up a tank of dirty water and if you won't want to drink dirty water, then you shouldn't want to eat those oysters after they have removed that gunk from the water. It's like eating a dirty air filter, so once you understand their role in the environment, then you see that they weren't intended to be eaten as food. God's law was given for our own good (Deuteronomy 6:24, 10:12-13), and there are a enormous number of diseases and parasites that have been transferred to humans through eating unclean animals, so God prohibited eating them for our own good because they weren't intended to be eaten as food, though God associated it with His eternal holiness, so there is more to it than that.
In Genesis 9:3, Noah was permitted to eat clean animals. It is not explicitly stated by the Bible, but I don't see a reason why when Adam and Eve were permitted to kill animals for clothing that they weren't also permitted to kill them for food. Furthermore, un Genesis 4:4, Cain brought the firstborn of his flock and the fat portions, and the primary reason for raising a flock is for food, so I don't think that Genesis 9:3 was granting something new, but was just lifting the temporary prohibition of Genesis 6:21.
In Genesis 6:8-9, it says that Noah found grace in God's eyes, that he was a righteous man, and that he walked with God. It was not the case that what Noah happened to be doing was righteous, rather in Psalms 119:1-3, God taught how to walk in His way through His law, and in Psalms 119:29-30, David wanted to put false ways far from him, for God to be gracious to him by teaching him to obey His law, and he chose the way of faithfulness, so this has always been the one and only way to be declared righteous by grace through faith. In Genesis 6:8-9, God was gracious to Noah by teaching him how to walk in His way in obedience to His law and he was a righteous man because he obeyed through faith. In Exodus 33:13, Moses wanted God to be gracious to him by teaching him His way that he might know Him, and the way to know God is by walking in His way in obedience to His law. In Genesis 18:19, Abraham taught His children and those of His household to walk in God's way by doing righteousness and justice, so it is all the same eternal law in accordance with the same eternal nature.
