I have news for you. At both Baptists and Pentecostal Bible school, I was taught the 10 commandments still apply. Not in the sense that that our salvation is dependent on them.
But that they define what sin is. I also attended a Lutheran church for several years and I assure they believe the 10 commandments still apply.
But I digress. They don't focus so much on each individual thing... i.e. don't covet, don't bear false witness, etc... as the total being... "love one another" (as Jesus put it).
Some sins you know by experience. If you've ever been lied to, or stolen from, you know it hurts. Some take this to mean "they are written on your heart".
Other sins, such as adultery and homosexuality may not have been taught to you by past experience. We know these are wrong because of the Bible.
Rom 3:20; because by the works of the Law no flesh will be justified in His sight; for through the Law comes the knowledge of sin.
Without the law, we wouldn't know certain things are sin.
Some people argue that the "knowledge of good and evil" is passed on from generation to generation. If this were true, we wouldn't have different opinions of what sin is.
In the state I live in, abortion, same sex marriage, and viewing pornography that exploits children is legal. Over a million people voted that they wanted these things.
Does that mean they aren't sin? Who defines what sin is, man or God. Does God lower his standards to meet what the flesh and satan want?
I think we can know sin in 3 ways: written law, experience of sin, knowledge of other's sin.. and God's nature within us.
Even if we know that certain things are sin due to the written law, it does not ensure that we truly know they are sins in our experience. We can know that something is a sin in our mind, but it may not be fully realized in our heart...and it is in the heart that really matters.
It is true that the commandments give us revelation so that we know what sin is. But this does not mean that if we don't have the written law we will not know what sin is. If this is the case then God cannot judge anyone who lived prior to Moses or anyone who does not have a bible or internet access. But the bible says are all without excuse (Rom 1:20). Sin is not only revealed by the written law, but also by the conscience and revealed nature of God in creation.
Sin can be taught by our own experience or the experience of others. But we don't have to commit a sin before we know it is wrong. As Christians we have received the life and nature of God, and so by His nature in us, we will know those things that please Him and those that don't.. as it says we can "have the mind of Christ" 1 Cor 2:16.
As the experience with the Pharisees shows.. merely teaching and preaching the law and having the written law does not ensure the keeping of the law or the knowledge of God.
In as much as the 10 commandments coincide with God's moral law, in that sense yes they still apply. All except the one about keeping the Sabbath. I've always found it strange how we quote the 10 commandments but ignore 1 of them. Keeping the Sabbath is not part of God's moral law. So is it 10 -1 = 9 commandments for Gentiles. God's moral law is spiritual and written on the heart, the Law of Moses is of letters written on stone tablets, or paper or iPads (these days).
I believe many denominations do not understand the law of the Spirit of life and living in the Spirit. The Holy Spirit is like the forgotten 3rd person of the Trinity. Can we find anywhere in the book of Acts where it says as a command, to love God and love neighbor as ourself? Despite all of the ministry to the Jew and Gentile recorded in Acts, it should be there.. but I know for a fact you won't be able to find it there. Do you know why? I think because no one had to be told to love God or love their neighbor. 1 John 4:19 says "we love Him because He first loved us". God loved us by sending His Son. Love is no more an explicit written commandment that has to be taught from generation to generation, but an implied consequence of receiving God's love by accepting Christ.