Execute the following sample apex code to get all the Salesforce Data Categories using Apex.

Sample Code:

private List < DataCategory > getAllCategories(
    List < DataCategory > categories
) {

    if ( categories.isEmpty() ) {

        return new List < DataCategory >();

    } else {

        List < DataCategory > categoriesClone = categories.clone();
        DataCategory category = categoriesClone[ 0 ];
        List < DataCategory > allCategories = new List < DataCategory > { 
            category 
        };
        categoriesClone.remove( 0 );
        categoriesClone.addAll(
            category.getChildCategories()
        );
        allCategories.addAll(
            getAllCategories(
                categoriesClone
            )
        );
        return allCategories;

    }

}

List < DescribeDataCategoryGroupResult >  describeCategoryResult;
List < DescribeDataCategoryGroupStructureResult > describeCategoryStructureResult;
List < String > objTypes = new List < String >() ;
objTypes.add ( 
    'KnowledgeArticleVersion' 
);
List < DataCategoryGroupSobjectTypePair> typePairs = 
    new List < DataCategoryGroupSobjectTypePair >();
describeCategoryResult = Schema.describeDataCategoryGroups( 
    objTypes 
);

for ( DescribeDataCategoryGroupResult groupResult : describeCategoryResult )  {
    
    DataCategoryGroupSobjectTypePair typePair =
        new DataCategoryGroupSobjectTypePair();
    typePair.setSobject( groupResult.getSobject());
    typePair.setDataCategoryGroupName(
        groupResult.getName()
    );
    typePairs.add( 
        typePair 
    );
    
}

describeCategoryStructureResult = 
    Schema.describeDataCategoryGroupStructures(
        typePairs, false
    );

for ( 
    DescribeDataCategoryGroupStructureResult structRes : 
    describeCategoryStructureResult 
) {
    System.debug(
        'Structure Result Name is ' + 
        structRes.getName()
    );
    
    DataCategory [] toplevelCategories = 
        structRes.getTopCategories();

    List < DataCategory > allCategories = 
               getAllCategories( toplevelCategories );
    
    for ( DataCategory objDC : allCategories ) {
        
        DataCategory [] childCategories = 
            objDC.getChildCategories();
        
        for ( DataCategory objCC : childCategories ) {

            System.debug(
                'Name is ' + 
                objCC.getName()
            );

        }
        
    }
    
}