Is Deuteronomy 6:8-9 literal?
As quoted in the original post, the scripture is referring to phylacteries, or scripture-containing cases, that were to remind the Jews of God's instructions about their deliverance. The scripture was not to be literal as is alluded to in Exodus 13:9
And this will serve for you as a sign on your hand and as a memorial on your forehead, so that Jehovah’s law may be in your mouth, for with a mighty hand Jehovah brought you out of Egypt. (NWT)
Jesus mentions these phylacteries or cases when speaking about how the Scribes and Pharisees wanted to be seen as more pious than other Jews:
All the works they do, they do to be seen by men, for they broaden the scripture-containing cases that they wear as safeguards and lengthen the fringes of their garments.–Matthew 23:5 (NWT)
Actual cases have been found in caves in Qumran
Qumran Phylactery at Bible Land Pictures
(For more information on these phylacteries see the article Frontlet Band and Scripture-Containing Case in the Insight On The Scriptures)