(Stuff you want vs don’t want) you expect to grow.

If you have A, B and C and know that you will not want D and E, do IN (‘A’, ‘B’).

If you know you will want to keep D and E then do NOT IN C.

You also need to be mindful of null values. SQL Server has a global setting that will control if nulls are considered NOT IN or skipped and return a null value. That’s likely why the guy wants to use both - but it’s better to coalesce and make sure there’s no nulls or control for them separately to be explicit