John Gill Commentary Deuteronomy 31:4

"And Jehovah will do unto them as he did to Sihon and to Og, the kings of the Amorites, and unto their land; whom he destroyed." — Deuteronomy 31:4 (ASV)

And the Lord shall do to them as he did to Sihon, and to
Og, kings of the Amorites
Deliver them up into their hands; see the history of this in (Numbers 21:10–35) ;

and to the land of them whom he destroyed ;
put them into the possession of the land of Canaan, as they were now in possession of the land of those two kings he destroyed by them. This instance is given to encourage their faith, assuring them that what has been done to them would be done to the Canaanitish kings, and their subjects, and their lands.