In keeping with the mood that 'that part of Law which is applicable is the part which is does not fall under the rule of obsolescence' then I want to examine clothing.
A major critic of mine has recently countered me by asking if I did the dietary laws. In fact, I abstain from unclean meats. That isn't the same as keep[ing dietary law, one law also states to eat only unleavened bread during Passover, which I will have to start next year. So I'm trying. He follows up by asking what about clothing weaved from two different materials. I do not do that. But I have not done it because I rarely ever go shopping and I have a lot of clothes that I've warn for many years and don't mean to stop. So as I start replacing things gradually as materials get old and fall apart, I will very seriously consider buying only those sorts of clothing. He, because he is warped and demented, thinks he has won the argument and shown that I am clearly a buffoon. But he hasn't won at all and this will explain why.
The Law's Purpose
These laws actually had very clear purposes. The land of Israel as we know today is quite small. The land of Israel in those days was larger but how much larger I'm not informed of those numbers but we'll suppose according to the maps seen regularly at the back of bibles it wasn't substantially larger in those days than it is now. Israel in that time was a quite fertile land, really a jewel of the Mid-East. Why, it seems perfectly logical that when you're sitting on a continent made up almost strictly of desert and arid climate, that you would want to implement proper agricultural practice so as to maintain the richness and fertility of the terrain you live on. Quite clever, Jehovah! And quite stupid for the humans who come along and think this is something to laugh at and mock.
Was this law designed to benefit the people of that region and in that time? Absolutely. Does it present a moral weight to us today? I don't think so. Because this overlooks exactly what is the type of soil of North America (that is, if you live in North America) and what are the techniques that are going to best consider what type of soil, what type of crops will be sustained, and what techniques are going to yield a bountiful crop. Maybe agricultural practice of Judea from 3000 years ago isn't best suited to the climate and geography of North America.
Nevertheless, if it helps my witness, and it puts my money where my mouth is, I will wear the proper clothing. Because I cannot make myself vulnerable on such a minimal point so that my most petty, belligerent, dim-witted adversaries can mock my ministry and say, "Hehe, he wears a cotton-polyester fabric!" I will simply choose to wear cotton and it feels much nicer to the touch so stick that in your pipe and smoke it. And if you're not a smoker you can just stick it.
To me, this is apparently something that falls under the rule of obsolescence but that I'd say if you were a farmer in Israel I think the law has continuing regional applicability unless the Jews have discovered some agricultural technique which supersedes the command in the Bible. And that's one aspect of obsolescence. For instance with hygiene and sanitary regulations it makes most sense that those laws are made obsolete by advances in terms of water treatment, sewer facilities, anti-septics and anti-bacterials, and all the many advances of Western culture which are more effective at eliminating the spread of disease than Levitical law. Those laws had definite value in that day and age but in a lot of ways, if not all ways, has been set aside in favor of modern medicine and science. So the law, if it applies at all, only applies to people in the state of Israel and nowhere outside of Israel. The law had a specific applicability and value in that era for that people and for us today does not serve the same value, but I will not say serves no value because I am not an agriculturist and would have no way to know that. I know if you're living on the only piece of tenable land in a radius of literally hundreds of miles and in some directions thousands of miles, and the LORD has said I will make you a great nation but you need to do this, this, and this, in terms of your crops, I would listen to Him, lest my land go to rot like every other pile of sand in that area. Will God burn me if I do something else with my fields? I don't think so. But if I go to plow land in Israel, I have a different take there.
Addendum: "We do not inherit the earth from our ancestors; we borrow it from our children." The Law teaches a conservation of land, if not for the purpose of contemporaries who will benefit, but so as to be protected for future generations. That would be sad for one generation to leave a desolated earth to its children.