Any idea what is included in this OpenSearch update OpenSearch_2_5_R20230928?

AWS Opensearch service doesn't disclose what is included in a specific service software patch. I wish they did. On the other hand, you have an option to not to install optional service software patches on the cluster which will only install the mandatory security updates. [] Service software updates in Amazon OpenSearch Service - Optional versus required updates - https://docs.aws.amazon.com/opensearch-service/latest/developerguide/service-software.html#service-software-optional-required