AFAIK Netflix Conductor was inspired by the AWS Simple Workflow engine. The Cadence is a direct evolution of SWF in the open source world. I was tech lead for both of them :).

You may also want to look at Project Flogo (flogo.io) - in particular, the Flogo Flow action. It's a process engine in Golang that can be used to implement service orchestration, event-driven choreography integration patterns.

Flogo has been focused more on stateless service choreography running in Serverless FaaS or Edge devices - however the underlying process engine can be used to implement a long running orchestration process with fully externalized state for resilience and scale. It's 3-clause BSD including the modeling UI with hundreds of triggers and activity extensions in the community. We have a fairly active gitter channel in case you have questions for the core team (https://gitter.im/project-flogo/Lobby)

Disclaimer: I work at TIBCO & we provide commercial support for Project Flogo as well as use it in our commercial products.

I guess technically Oozie is also a FOSS Java-based workflow/orchestration engine, except it's mostly for Hadoop jobs. Haven't used too many orchestration engines but I found Oozie very barebones