It should be usefull to be able to group element in legend by unique symbol.
Exemple :

I would like to group "Voies d'une longueur 19", and "Voies de plusieurs longueurs 7" in a new one " Voies 26"
and "Bloc 5" and "Psychobloc 1" in a new one "Blocs 6"
It's possible in ArcGIS Pro, should be nice and usefull in Arcgis Online 🙂