ScriptCode field in P2SH-P2WPKH BIP-143 Test Vector
The one that you are referring to is sighashing. There is no good reason to use 1976a91479091972186c449eb1ded22b78e40d009bdf008988ac (dup hash160 [79091972186c449eb1ded22b78e40d009bdf0089] equalverify checksig as the scriptCode in Segwit but to limit the number of changes SegWit makes to the original sighashing. In designing SegWit, only intended changes were made. The hash structure remains the same, and contains all the same data. The sighash flags still have the same effect. The only thing that changes in BIP-143 is that some data is organized under precomputed hashes, and the input amounts go into it.