This is how to make your custom theme to include extra suggestions based on the term vocabulary name and how to set a default value for the checkbox if you are on the taxonomy/term-id page.
function my_theme_suggestions_input_alter(&$suggestions, array $variables) {
if ($variables['element']['#type'] === 'checkbox') {
if (isset($variables['element']['#parents'][0])) {
$suggestions[] = $variables['theme_hook_original'].'__'.$variables['element']['#parents'][0];
}
}
}
Now this will provide a suggestion based on the custom entity reference name field.
e.g. input--checkbox--field-vocabulary-taxonomy--ref-target-id.html.twig
Next we need to see if the current taxonomy term page matches the current input checkbox exposed views filter.
function my_theme_preprocess_input__checkbox(&$variables) {
if (\Drupal::routeMatch()->getRouteName() == 'entity.taxonomy_term.canonical') {
$term_id = \Drupal::routeMatch()->getRawParameter('taxonomy_term');
if ($variables['attributes']['value'] == $term_id) {
$variables['attributes']['checked'] = 'checked';
}
}
}
And that's it. When you visit /taxonomy/term-id page your exposed filter checkbox will get the default value from the term-id url arg.