That is: are packets silently dropped, or does the firewall respond with a RST/ACK (tcp) or ICMP Port Unreachable packet?
I ran some experiments here: https://github.com/turtlemonvh/aws-network-experiments
Traffic denied by either AWS security group rules and network ACLs behaves identically to traffic encountering an iptables
DROP rule (vs
REJECT --reject-with tcp-reset). No response is sent in either case.