Is it possible to still Jira Workflow Engine as a backend service for a different commercial product? If so, where can I find information about licensing.
Not directly. The "platform" is coded in a way that means workflows need to work with Jira issues (and hence screens, events, fields, and so-on).
If you just want the workflow engine part of Jira, you don't actually need Jira at all - the engine is an open source thing - see https://developer.atlassian.com/server/jira/platform/workflow-modules/
Your options here would be
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.