Only Gold and Platinum subscriptions have access to uploading custom plugins. All subscription levels, including Standard, can upload scripts and dictionaries.
There are several cases where you might need your own files to be made available to your Elasticsearch cluster’s nodes:
To facilitate this, we make it possible to upload a zip file that contains the files you want to make available. Uploaded files are stored using Amazon’s highly available S3 service. This is necessary so we do not have to rely on the availability of third-party services such as the official plugin repository when provisioning nodes.