When I was using Deconz, there was an attribute is_deconz_group.
For zigbee2Mqtt groups, I don’t find an attribute like is_mqtt_group in ha. Why?
Because MQTT itself doesn’t have the concept of a group. I.e. mqtt just has topics. You can create a device or entities. Nothing more.
If you want to create a mqtt entity with that attribute, you can add it yourself by manually configuring an entity to have that attribute.
I’m talking about this:
https://www.zigbee2mqtt.io/guide/usage/groups.html#configuration
Update: it’s almost implemented in Z2m.
Then this WTH will be closed and you should request that attribute in Zigbee2mqtts repo. The values for MQTT entities are provided by discovery topics and their contents. Zigbee2mqtt provides that information to home assistant.
To clarify further, they will be providing a json_attributes_template in the discovery topic which tells home assistant what attributes to create. This needs to be provided by them, not HA.