Hello,
I do not believe that any of the rules in the OT are defunct, but that God's ways are eternal, though sadly my view is not mainstream. However, it is important to keep in mind that even when the Law was first given to Moses, there was not a single person who was required to obey every single law, and not even Jesus obeyed the laws in regard to a woman's period or to giving birth. Some laws were only for the King, High Priest, priests, judges, men, women, children, widows, people living in the land, strangers living among them, and for everyone. Furthermore, many of the laws come with conditions under which they apply, such as the Sabbath, which only applies when it is the 7th day, or to laws in regard to temple practice that only apply when there is a temple in which to practice them.
There is also the problem that the laws were given to a theocratic nation, so it becomes an issue of who has jurisdiction when we have citizenship in a different nation. Even when Jews were under a theocratic nation, they did not have a history of executing someone every time the Law prescribed it, but rather they often imposed a fine instead. The harsher penalty was to show the seriousness of the sin, whereas the light penalty was to show the mercy of God in the hope the person would repent from their actions. In Matthew 23:23, Jesus said that mercy was one of the weightier matters of the Law, so if mercy is not evident in how we are obeying it, then we are not obeying it correctly.
In
John 8:1-12, it recounts an incident with Jesus and a woman accused of adultery, but this is example of Jesus following the Law rather than making changes to how it is obeyed. There was no judge to pronounce a sentence (
Deuteronomy 19:17-21), there was no man accused (
Leviticus 20:10), he didn't have any witnesses to examine (
Numbers 35:30,
Deuteronomy 19:5), and he did not have a confession, so if he had condemned her, then he would have acted in violation of the Law. Just a few verses later Jesus said that he judged no one (
John 8:15) and he also said that he came not to judge (
John 12:47), so he did not exercise authority as a magistrate and did not condemn her, but he did recognize her action as sin, and told her to go and sin no more. In any case, Jesus gave himself to pay the penalty for our sins, so it would not be appropriate to to enforce a penalty that has already been paid, but I do think that Christians are required to refrain from eating unclean animals, such as shellfish.