[Standards] Re: XEP-0503: Spaces update proposal

I mean it's nothing to do with MUCs or MUC services. disco#items is a generic hierarchical federated "lists of things" protocol which can include anything including but not limited to MUCs (since they have JIDs) or PubSub nodes (since they have JID + node), and also "other lists" (hence hierarchical). Most clients for decades have had a UI to browse any such hierarchical structures. The original draft of this XEP documented this existing protocol for people who might not have realised its relation to the stated use case. Yes it's true that a "MUC service" also implements this protocol, and that in practise it often lists only public MUCs (though this can and is being changed) and that as a result a "MUC service" happens to conform to the original draft (giving us existing browsable compliant spaces like xmpp.org and joinjabber.org) however this doesn't mean that using disco#items requires MUCs or MUC services at all.