I thought I should add more details.
Here are my investigations :
There is a screen capture of my post edition page attached.
I found a json file in my theme that seems to deal with categories in an ACF-jon folder :
{
"key": "group_5ac5d1e87ab94",
"title": "Categorie",
"fields": [
{
"key": "field_5ad704be7333f",
"label": "Afficher la catégorie courante",
"name": "afficher_les_articles_dans_la_page_categorie",
"type": "true_false",
"instructions": "",
"required": 0,
"conditional_logic": 0,
"wrapper": {
"width": "",
"class": "",
"id": ""
},
"message": "",
"default_value": 1,
"ui": 1,
"ui_on_text": "",
"ui_off_text": ""
}
],
"location": [
[
{
"param": "taxonomy",
"operator": "==",
"value": "categorie"
}
]
],
"menu_order": 0,
"position": "normal",
"style": "default",
"label_placement": "top",
"instruction_placement": "label",
"hide_on_screen": "",
"active": 1,
"description": "",
"modified": 1530175641
}
And finally that’s what I tried to add in function.php with no success :
add_filter('body_class', 'add_acf_body_class');
function add_acf_body_class($class) {
$value = get_field('afficher_les_articles_dans_la_page_categorie', false, false);
$class[] = $value;
return $class;
}