channel_leave message event
A member left a channel
{
"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.