message

{
    "type": "message",
    "subtype": "thread_broadcast",
    "text": "broadcasting this reply",
    "user": "U123ABC456",
    "ts": "1673464745.620769",
    "thread_ts": "1673464730.703009",
    "root": {
        "client_msg_id": "123abc456-...",
        "type": "message",
        "text": "This is the original message",
        "user": "U123ABC456",
        "ts": "1673464730.703009",
        "blocks": [
            {
                "type": "rich_text",
                "block_id": "qTg",
                "elements": [
                    {
                        "type": "rich_text_section",
                        "elements": [
                            {
                                "type": "text",
                                "text": "This is the original message"
                            }
                        ]
                    }
                ]
            }
        ],
        "team": "T123ABC456",
        "thread_ts": "1673464730.703009",
        "reply_count": 1,
        "reply_users_count": 1,
        "latest_reply": "1673464745.620769",
        "reply_users": [
            "U123ABC456"
        ],
        "is_locked": False
    },
    "blocks": [
        {
            "type": "rich_text",
            "block_id": "BVp",
            "elements": [
                {
                    "type": "rich_text_section",
                    "elements": [
                        {
                            "type": "text",
                            "text": "broadcasting this reply"
                        }
                    ]
                }
            ]
        }
    ],
    "client_msg_id": "123abc456-...",
    "channel": "C123ABC456",
    "event_ts": "1673464745.620769",
    "channel_type": "channel"
}

The thread_broadcast message subtype is sent when a user or bot user has indicated their reply should be broadcast to the whole channel.

It's a pointer or reference to the actual thread and is meant more to be informational than to fully describe the message. The reference cannot contain attachments or message buttons.

The message includes a root field with a modified form of the original message in the thread, meant for use in rendering by Slack client applications.

See message threading for more information.