There is an example on how to close a channel (i.e. websocket) server-side in the docs (Phoenix.Socket — Phoenix v1.8.8)
MyAppWeb.Endpoint.broadcast("users_socket:" <> user.id, "disconnect", %{})
But AFAICS it always needs those ids, which you could gather using presence track which keeps its updates (presence_diff) private.
On a shutdown signal you could iterate these ids and broadcast that above disconnect to your sockets.
