The sse:id collection is for every unique SSE subscription created when clients connect or reconnect to the SSE /event-stream. They’re normally cleaned up as part of the normal operation as client connections are dropped off but if the AppDomain recycles they may not get a chance to remove themselves.
There is a Reset() API on IServerEvents you can use to flush all local connections and Redis SSE metadata subscriptions:
container.Resolve<IServerEvents>().Reset();
You can try creating a Service that calls this API on all AppServers (at the same time) when the App Servers are locked which will flush the local SSE connections and redis SSE metadata. Calling it after deployment to all AppServers is another good time to call it to remove any zombie metadata.