I saw that too.
On the Amazon SNS console I already had two “Topics” set up. I have only hazy memories of setting up SES. It must have been in the relevant Discourse instructions. It’s also mentioned in this recent topic: How do I properly configure the AWS SNS topic ARN allowlist?.
One was for bounces and one for complaints, and there were ARNs for each in the format arn:aws:sns:eu-west-1:111111111111:DiscourseBounces. I put both of these ARNs into the aws_sns_topic_arn_allowlist Discourse setting and the warning doesn’t appear any more. I hope that means I did the right thing…