Many modern devices support the Zigbee 3 protocol, which allows you to use group binding to control multiple devices with one command.
The direct binding is faster than event-based interaction and eliminates the “popcorn” effect when the devices act one after the other in random order.
Does ST have any plans to support Zigbee group messages?
Just to add a historical note: both ZHA 1.2 and ZLL also supported zigbee groups for both lights and sensors. Not all implementations supported grouping for HVAC devices (NXP famously did not) but otherwise it was very similar to Zigbee 3.0 with regard to groups.
https://www.nxp.com/docs/en/user-guide/JN-UG-3076.pdf#page77
So it’s always been a mystery as to why smartthings did not fully support zigbee groupcasting. 
. I have sometimes wondered if someone made the architecture decision that they were going to require ACK for all messages. or if they were concerned about traffic, since in zigbee a groupcast is actually sent to every device on the network, who examines it to see if they are in the group to which it is addressed.
In any case, I definitely agree that better support for zigbee 3.0 groups would be an excellent feature.
Last I heard, Lighting Groups use Zigbee group casting (assuming all the device in the group are zigbee). Of course Lighting Group have been hobbled for years by a poor implementation (can’t put in scenes or automations), but maybe that will change one day.
edit: I think they do support Groups in Edge drivers:
https://developer-preview.smartthings.com/edge-device-drivers/zigbee/generated/clusters/Groups.html
Yes, as you mention the group messages for Zigbee are not currently supported. However, I already mentioned this to the corresponding team and they are aware of this request.
I suggest you stay tuned to the newsletter (at the bottom) and the announcements where this kind of feature is announced.
Also, the Developer documentation Release Notes are an excellent option to stay up-to-date on what’s new there: