Orion aims to provide compatibility for plugins. Plugins written for previous versions of Orion, that only use well documented services, will continue to work in Orion 13.0.
Documented server REST API has only changed in compatible ways. Existing client code running against previous versions of the Orion server (either Java or Node) will continue to work in Orion 13.0.
No compatibility is provided within Orion server code, on either Java or Node servers.