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hook_preprocess_menu is missing a lot of variables that other preprocessors have so I finally solved my problem using hook_preprocess_block (which I think would not have been possible in earlier versions of drupal)

function THEMENAME_preprocess_block(&$vars){
    if($vars['base_plugin_id'] == "system_menu_block"){
        if($vars['derivative_plugin_id'] == "main"){           
            if($vars['configuration']['region'] == "navbar"){
                $vars['content']['#attributes']['class'][] = 'in-navbar';
                $vars['content']['#attributes']['class'][] = 'nav';
            }
            elseif($vars['configuration']['region'] == "footer"){
                $vars['content']['#attributes']['class'][] = 'in-footer';
            }
        }
    }
}

NOTE if using in a recursive function/twig MACRO (which probably you are) you should removeClass all the classes you don't want in inner UL tags in a way similar to this

{{ menus.menu_links(item.below, attributes.removeClass('nav','in-navbar'), menu_level + 1) }}

hook_preprocess_menu is missing a lot of variables that other preprocessors have so I finally solved my problem using hook_preprocess_block (which I think would not have been possible in earlier versions of drupal)

New solution since people have been reporting that $vars['configuration']['region'] is empty

function THEMENAME_preprocess_block(&$vars){
    if($vars['base_plugin_id'] == "system_menu_block"){
        if($vars['derivative_plugin_id'] == "main"){           
            $block = \Drupal\block\Entity\Block::load($vars["elements"]["#id"]);
            if( $block->getRegion() == "navbar")
                $vars['content']['#attributes']['class'][] = 'in-navbar';
                $vars['content']['#attributes']['class'][] = 'nav';
            }
            elseif($vars['configuration']['region'] == "footer"){
                $vars['content']['#attributes']['class'][] = 'in-footer';
            }
        }
    }
}

NOTE if using in a recursive function/twig MACRO (which probably you are) you should removeClass all the classes you don't want in inner UL tags in a way similar to this

{{ menus.menu_links(item.below, attributes.removeClass('nav','in-navbar'), menu_level + 1) }}

old solution

function THEMENAME_preprocess_block(&$vars){
    if($vars['base_plugin_id'] == "system_menu_block"){
        if($vars['derivative_plugin_id'] == "main"){           
            if($vars['configuration']['region'] == "navbar"){
                $vars['content']['#attributes']['class'][] = 'in-navbar';
                $vars['content']['#attributes']['class'][] = 'nav';
            }
            elseif($vars['configuration']['region'] == "footer"){
                $vars['content']['#attributes']['class'][] = 'in-footer';
            }
        }
    }
}

hook_preprocess_menu is missing a lot of variables that other preprocessors have so I finally solved my problem using hook_preprocess_block (which I think would not have been possible in earlier versions of drupal)

function THEMENAME_preprocess_block(&$vars){
    if($vars['base_plugin_id'] == "system_menu_block"){
        if($vars['derivative_plugin_id'] == "main"){           
            if($vars['configuration']['region'] == "navbar"){
                $vars['content']['#attributes']['class'][] = 'in-navbar';
                $vars['content']['#attributes']['class'][] = 'nav';
            }
            elseif($vars['configuration']['region'] == "footer"){
                $vars['content']['#attributes']['class'][] = 'in-footer';
            }
        }
    }
}

NOTE if using in a recursive function (which probably you are) you should removeClass all the classes you don't want in inner UL tags in a way similar to this

{{ menus.menu_links(item.below, attributes.removeClass('nav','in-navbar'), menu_level + 1) }}

hook_preprocess_menu is missing a lot of variables that other preprocessors have so I finally solved my problem using hook_preprocess_block (which I think would not have been possible in earlier versions of drupal)

function THEMENAME_preprocess_block(&$vars){
    if($vars['base_plugin_id'] == "system_menu_block"){
        if($vars['derivative_plugin_id'] == "main"){           
            if($vars['configuration']['region'] == "navbar"){
                $vars['content']['#attributes']['class'][] = 'in-navbar';
                $vars['content']['#attributes']['class'][] = 'nav';
            }
            elseif($vars['configuration']['region'] == "footer"){
                $vars['content']['#attributes']['class'][] = 'in-footer';
            }
        }
    }
}

NOTE if using in a recursive function/twig MACRO (which probably you are) you should removeClass all the classes you don't want in inner UL tags in a way similar to this

{{ menus.menu_links(item.below, attributes.removeClass('nav','in-navbar'), menu_level + 1) }}

hook_preprocess_menu is missing a lot of variables that other preprocessors have so I finally solved my problem using hook_preprocess_block (which I think would not have been possible in earlier versions of drupal)

function THEMENAME_preprocess_block(&$vars){
    if($vars['base_plugin_id'] == "system_menu_block"){
        if($vars['derivative_plugin_id'] == "main"){           
            if($vars['configuration']['region'] == "navbar")
                $vars['content']['#attributes']['class'][] = 'in-navbar menu nav';
            elseif($vars['configuration']['region'] == "footer")
                $vars['content']['#attributes']['class'][] = 'in-footer';
        }
    }
}

and those classes go into the menu parent element (UL)

hook_preprocess_menu is missing a lot of variables that other preprocessors have so I finally solved my problem using hook_preprocess_block (which I think would not have been possible in earlier versions of drupal)

function THEMENAME_preprocess_block(&$vars){
    if($vars['base_plugin_id'] == "system_menu_block"){
        if($vars['derivative_plugin_id'] == "main"){           
            if($vars['configuration']['region'] == "navbar"){
                $vars['content']['#attributes']['class'][] = 'in-navbar';
                $vars['content']['#attributes']['class'][] = 'nav';
            }
            elseif($vars['configuration']['region'] == "footer"){
                $vars['content']['#attributes']['class'][] = 'in-footer';
            }
        }
    }
}

NOTE if using in a recursive function (which probably you are) you should removeClass all the classes you don't want in inner UL tags in a way similar to this

{{ menus.menu_links(item.below, attributes.removeClass('nav','in-navbar'), menu_level + 1) }}