1. Open network, but each user lives on a single server, each conversation is dependent on a single server: (XMPP MUCs)

2. Open network, but each user lives on a single server (with some ability to manually migrate between servers), conversations are replicated across all participating servers: (Matrix, ActivityPub, SMTP, NNTP)

3. Open network, users are replicated across multiple servers, conversations are replicated across all participating servers: (Matrix + MSC1228 or MSC2787 or similar)

4. Open network, users live on a single P2P node, conversations are replicated across all participating nodes: (Briar, today's P2P Matrix)

5. Open network, users are replicated across multiple P2P nodes, conversations are replicated across all participating nodes: (P2P Matrix + MSC2787 etc).

So yes, XMPP is decentralised by some definition, but it's kinda useful to map out the whole space.