public boolean onRequiredStructureOrUnion() {
            return onRequiredStructureOrUnion;
        }
Whether clientOptional is required for members marked as required that
target structures or unions (shapes with no zero value, meaning it's
impossible to later remove the required trait and replace it with the
default trait).

@return Returns true if required.
smithy-linters/src/main/java/software/amazon/smithy/linters/MissingClientOptionalTrait.java