Comment by nlitened

> I want to represent that a field should never get new null values, it should be treated as non-nullable for the purpose of writing

That's a job of DB triggers — arbitrary restrictions that are automatically applied when you create or update records. You can restrict new null values, you can restrict numeric values to be odd or prime, you can make sure that only a single field can be null out of two, but not two at the same time — all the things that you can't even describe in a type system.