That's not entirely true, a router with a full PnP stack would listen to and participate in SSDP discover queries, enabling more discoverable services per host.

That's not entirely true, a router with a full PnP stack would listen to and participate in SSDP discover queries, enabling more discoverable services per host. Only one service can listen on port 1900 for DISCOVER queries, but a SSDP-capable router can cache NOTIFY messages from services that can't listen, and reply to queries for them itself with the same record. It's admittedly a weak point to make in the context of consumer gear, though. Last I checked, even router OS's with sprawling functionality like OpenWRT and PFsense don't offer SSDP relaying as a supported feature. If you spent less than $200 on your router, there is probably no difference between disabling all of its UPnP capability and disabling IGDP specifically.