In a P2P network where not all nodes subscribe to the same topic, if a message reaches a node and the node is not subscribed to the topic the message is intended for, it drops the message when using GossipSub. This means that to ensure 100% message dissemination, all nodes subscribed to a particular topic must be eventually connected to all other nodes on the same topic. If there are several different topics dividing the nodes in disjoint sets, this results a large fanout (a very dense network).
This severely reduces how well we can scale since if there are more than 1000s of nodes we end up with each node having several hundred direct connections to ensure full connectivity for each topic. (I had asked this question earlier as well - Gossip sub IWANT issued only for local topics?)
In the new GossipSub spec v1.1. there is this new concept of
Explicit Peering Arrangements. With explicit peering arrangements, in a gossipsub setup, will the intermediate nodes propagate messages for topics they are not subscribed to?