It's not out of scope for the WG - in fact, the SocialWG is getting PubSubHubbub as an editor's draft soon, and ActivityPub (another WG spec) contains it's own subscription mechanism - and in part because of these, it's out of scope for the LDN spec in particular. There are lots of subscription mechanisms. We'd gladly accept help on assembling a list of the most applicable so we can point to them from LDN. But I don't know that we have the expertise to just pick one and say MUST or SHOULD do this. So far several people have suggeseted a subscription mechanism is useful, and everyone has offhandedly suggested something completely different..