C. F. Keil and Franz Delitzsch on Deuteronomy 12:20-21

Deu 12:20-21

These rules were still to remain in force, even when God should extend the borders of the land in accordance with His promise. This extension relates partly to the gradual but complete extermination of the Canaanites (Deu 7:22, comp. with Exo 23:27-33), and partly to the extension of the territory of the Israelites beyond the limits of Canaan Proper, in accordance with the divine promise in Gen 15:18. The words “as He hath spoken to thee” refer primarily to Exo 23:27-33. (On Deu 12:20, see Deu 12:15). - In Deu 12:21, “if the place...be too far from thee,” supplies the reason for the repeal of the law in Lev 17:3, which restricted all slaughtering to the place of the sanctuary. The words “kill...as I have commanded thee” refer back to Deu 12:15.