This is only relevant to pure languages. And this doesn’t come anywhere near to properly making that distinction anyway: a non-void function can have all the s

This is only relevant to pure languages. And this doesn’t come anywhere near to properly making that distinction anyway: a non-void function can have all the side effects, and a void function can have no side-effects. It also does not “make sure higher order code and side effecting code mix properly”, it just makes a subset of likely side-effecting code not mix with higher order code at all.