Is Genesis 32:32 a command or a historical account of a tradition?
New American Standard Bible: Therefore, to this day the sons of Israel do not eat the tendon of the hip which is on the socket of the hip, because he touched the socket of Jacob’s hip in the tendon of the hip.
King James Bible: Therefore the children of Israel eat not of the sinew which shrank, which is upon the hollow of the thigh, unto this day: because he touched the hollow of Jacob's thigh in the sinew that shrank.
It is a memoir and it is a memoir of man devising a tradition not of God issuing a command, indeed it is explicitly revealing that the custom of the Israelites on the eating of that sinew was a custom created by man rather than commanded by God.
Genesis is part of the Law, and we do find at least one commandment in it - circumcision - and this commandment is also given in the form of a historical event, Genesis 7:9-14. But the event in question in God issuing a command. And that is the general picture that we see in the Pentateuch where commandments are given within the narrative either by God directly, or by Moses. We also see God telling Moses to explain the teaching of circumcision to the Israelites (Leviticus 12:3), but see no mention of this sinew anywhere outside this Genesis narrative.
We do not have to sift through the Pentateuch to discover what things are commands and what are not - what are commands that apply to us is sometimes trickier - since commandments in the Pentateuch have a known commander - either God, or his legitimate agent Moses. Genesis 32:32 gives no such commander, it is a historical aside.
In short, Genesis 32:32 is not what a commandment looks like. It is explaining the origins of a custom that originates neither with the commandments of God nor the customs of foreigners, but the ancestral piety of the children of Israel.
Radak answers your question - that this an historical account of a tradition which later became a commandment:
על כן לא יאכלו בני ישראל, the children of Yaakov adopted this prohibition for themselves in commemoration of their father whose thigh muscle had been injured. They in turn commanded their children to abstain from eating this part of any animal. This continued until the Torah was given and it became law. It reminds us of the descendants of Yonadav ben Rechev who faithfully adhered to the clan’s founding father’s injunction not to drink wine (Jeremiah 35,6) in honour of the oath they swore to their forefather. They are described as loyalists par excellence in the Book of Chronicles. Moses recorded the custom of the Israelites not to eat this organ as one of the 613 commandments. Although the Torah does not generally consider it necessary to justify G’d’s legislation with a reason, in this case, since this law had been observed as a Jewish tribal custom for so many hundreds of years prior to the giving of the Torah, we are told how the custom which became enshrined in Torah law originated. Sefaria Radak on Genesis 32:32
