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It does not say written

The Law was written:

.9 So Moses wrote this law and gave it to the priests, the sons of Levi who carried the ark of the covenant of the Lord, and to all the elders of Israel. 10 Then Moses commanded them, saying, “At the end of every seven years, at the time of the year of remission of debts, at the Feast of Booths, 11 when all Israel comes to appear before the Lord your God at the place which He will choose, you shall read this law in front of all Israel in their hearing. 12 Assemble the people, the men and the women and children and [a]the alien who is in your [b]town, so that they may hear and learn and fear the Lord your God, and be careful to observe all the words of this law.
Deuteronomy 31:9-12

And not only was the law written but the the people are obligated to listen to it and obey it, and in this Word they can find life:

46 he said to them, “Take to your heart all the words with which I am warning you today, which you shall command your sons to observe carefully, even all the words of this law. 47 For it is not an idle word for you; indeed it is your life. And by this word you will prolong your days in the land, which you are about to cross the Jordan to possess.”
Deuteronomy 32:46-47

Cordially,

Your quote from Isaiah did not say written.

However, you quote from Deut 31 precedes the word law with the word this, not the. This leaves open the possibility that there is another law that is not written. I do not deny that there is a written law; however, I have seen nothing that states only written.