Some questions about Subscription collection

The rule applies to both cases for clients that POST to the subscription collection to subscribe to events. The clause you're referencing is specifically under "POST to subscription collection", so the statement does not apply to SSE; the client never performs a POST operation in the first place.

For more context with SSE, the presence of the subscription is an artifact of the fact that there is an active SSE stream between a client and service; the presence of the resource allows administrators to inspect who is listening for events. When the connection is dropped between the client and service, the service deletes the resource. Since a restart of the BMC is going to drop the connection, the same behavior applies.