During upgrade, older routers can still route config.system.sessions create requests to the config server because routers do not have FCV knowledge and are upgraded last. The config server currently preserves compatibility by forwarding those requests to the first shard, where the create collection coordinator runs.

Once 9.0 becomes last LTS, all supported routers will use the new routing behavior, so this compatibility path can be removed. Embedded config servers should continue to work through the normal local coordinator path.