Starting with 2.9.0 all gateway connections always use interest-only mode:

Phasing out of the "optimistic" mode whereby a server could send messages to the remote cluster without knowing if there was an interest or not. The remote cluster would reply with a "no interest" protocol. As of v2.9.0, servers that creates a gateway connection to a server of that version (and above) will no longer send messages in optimistic mode since it is assumed that all accounts will be switched to interest-only mode (where the subscription interest map is sent over) (https://github.com/nats-io/nats-server/pull/3383)