I think Slack is different than Gmail because people are actively having conversations, so if you disconnect, it is much more likely to be noticeable and annoying.
Reading between the lines, I think what they would need is a way to tell clients to move to a new websocket connection at the proxy layer. I don't think there is an easy built-in way to do this in the websocket protocol, so they would have to implement something custom in their application layer. This would also require triggering custom code in the client to make a new websocket connection, start using it, and then close the old connection.
I feel like it would have been simpler to just have the client do a graceful reconnect every 5 minutes. But they probably decided to use envoy so they could have the other advantages too.
Reading between the lines, I think what they would need is a way to tell clients to move to a new websocket connection at the proxy layer. I don't think there is an easy built-in way to do this in the websocket protocol, so they would have to implement something custom in their application layer. This would also require triggering custom code in the client to make a new websocket connection, start using it, and then close the old connection.
I feel like it would have been simpler to just have the client do a graceful reconnect every 5 minutes. But they probably decided to use envoy so they could have the other advantages too.