As pointed by @ mhoydis You can download the plugin zips directly using the URL in the manifest file present inside the OpenSearch distribution artifact. We are working on adding support to install these plugins via ./opensearch-plugin command as well. We will keep you updated on the progress and you can track the status here
In the meantime you can still use the ./opensearch-plugin command to install the plugins by passing the URL of the plugin zips present in the distribution manifest file
Example:
./opensearch-plugin install https://ci.opensearch.org/ci/dbc/distribution-build-opensearch/1.2.3/295/linux/x64/builds/opensearch/plugins/opensearch-alerting-1.2.3.0.zip