If the New Covenant has been made with us that was not made with the ancient Israelites and it contains some of the same laws, then the fact that a covenant was made with the ancient Israelites that was not made with their fathers does not mean that their fathers did not follow the same laws. For example, it is a sin to commit adultery under the New Covenant, under the Mosaic Covenant, and Joseph also knew that it was a sin to do that long before the Mosaic Covenant was made (Genesis 39:9).
In Matthew 4:15-23, Jesus began his ministry with the Gospel message to repent for the Kingdom of God is at hand, which was a light to the nations, and the Mosaic Law was how his audience knew what sin is (Romans 3:20), so repenting from our disobedience to it is a central part of the Gospel message, which Jesus preached in fulfillment of the promise to bless us by turning us from our wickedness (Acts 3:25-26), which was the Gospel that was made known in advance to Abraham in accordance with his promise (Galatians 3:8), and which he preached to those in Haran in accordance with the promise (Genesis 12:1-5).
There are many verses that describe the Mosaic Law as being instructions for how to walk in God's way, such as Deuteronomy 10:12-13, 1 Kings 2:1-3, Joshua 22:5, Isaiah 2:2-3, Psalms 103:7, Psalms 119:1-3, and many others. In Genesis 18:19, God knew Abraham that he would teach his children and those of His household to walk in His way by doing righteousness and justice that the Lord may bring to him all that He has promised. In Genesis 26:4-5, God will multiply the children of Abraham as the stars in the heaven, to his children He will give all of these lands, and through his children all of the nations of the earth will be blessed because Abraham heard God's voice and guarded His charge, His commandments, His statutes, and His laws. In Deuteronomy 30:16, if the Israelites will love the Lord their God with all of their heart by walking in His way in obedience to His commandments, statutes, and laws, then they will live and multiply and God will bless them in the land that they go to posses. So the promise was made to Abraham and brought about because he walked in God's way in obedience to His law, he taught his children and those of his household to do that, and because they did that in obedience to the Mosaic Law.
In John 8:39, Jesus said that if they were children of Abraham, then they would be doing the same works as him, so the way that the children of Abraham are multiplied in accordance with the promise is not through having physical descendants, but through teaching the nations to do the same works as Abraham by turning them from their wickedness and teaching them how to walk in God's way in obedience to His law in accordance with spreading the Gospel, which is also the means by which the children of Abraham are a blessing to the nations (Psalms 119:1-3). It would be contradictory to think that the children of Abraham are expected to being doing the same works as Abraham by walking in God's way while also thinking that God instructed the them to do something completely different than what He instructed to Abraham, so yes Abraham also kept the Sabbath holy. Keeping the Sabbath testifies that God created the world in six days, rested on the 7th, that He is holy, that He sanctifies us, and that He saves His people out of bondage, so only those who keep the Sabbath holy believe in the truth of these things.
