Having an article when it is grammatically required is imperative. Without a/an it is not only grammatically incorrect, but it also sounds really bad and will label you as a beginner English student, which you clearly aren't. This is the type of mistake that makes it into discriminatory ELL stereotypes.
Thankfully, this is one of the easier things to improve ...
This is simply an elided form of
We've improved the visual separation between the header and the body in classes with multiline headers.
Since an indefinite article would not make sense, the definite article can safely be omitted and understood, and people often do so. Each class has exactly one header and one footer, so there is no misunderstanding.