Text only applies to webhooks – how does this work for websockets? Easiest – each subscribe creates its own wss url. Unsubscribe to url closes it. A change in subscriptions would then be: subscribe to new set, close old one.

Existing Wording:

Hubs SHALL allow subscribers to re-request subscriptions that are already activated. Each subsequent and verified request to a Hub to subscribe or unsubscribe SHALL override the previous subscription state for a specific hub.topic, hub.callback / hub.channel.endpoint url, and hub.events combination. For example, a subscriber MAY modify its subscription by subscribing to or unsubscribing from additional events by sending subscription requests for additional events with the same topic and callback/endpoint url.