As I have seen in this video : https://www.youtube.com/watch?v=iD2kb51Q7xg

, the feature "Group by many2many field" in Tree view is available in Odoo 15:

I have already found pieces of code corresponding to this feature  in groupby_menu.js in the web module:

        _validateField(field) {
            return (field.type === "many2many" ? field.store : field.sortable) &&
                field.name !== "id" &&
                GROUPABLE_TYPES.includes(field.type);
        }

and in the module web_gantt, in the file gantt_model.js :

const isM2MGrouped = this.ganttData.fields[groupedByField].type === "many2many";
        let groupedRecords;
        if (isM2MGrouped) {
            groupedRecords = {};
            for (const [key, currentGroup] of Object.entries(currentLevelGroups)) {
                groupedRecords[key] = [];
                const value = currentGroup[0][groupedByField];
                for (const r of records || []) {
                    if (
                        !value && r[groupedByField].length === 0 ||
                        value && r[groupedByField].includes(value[0])
                    ) {
                        groupedRecords[key].push(r)
                    }
                }
            }
        } else {
            groupedRecords = groupBy(records || [], groupedByField);
        }

Has anyone advice to add this feature properly to odoo 13 or to purchase the corresponding module (or in OCA)?