message

{
    "type": "message",
    "subtype": "channel_leave",
    "ts": "1358877455.000010",
    "user": "U2147483828",
    "text": "<@U2147483828|cal> has left the channel"
}

A channel_leave message is sent when a member of a channel leaves that channel.

If you prefer a more atomic event, use member_left_channel instead. Consult the changelog for further guidance.