Workflow, Workflows, Workspace, and State Machine-- what is the difference? Drupal 8 core has an experimental module called Workflows.

There is also a contrib moduled called Workflow.

There's also the Workspace module, which is associated with the Deploy module.

Commerce 2 uses State Machine, another module that provides workflow management in Drupal 8.

What are the differences between these four modules?

Is one more future-proof than the others?  Normally, I would assume the module in core has the brightest future, but in this case Commerce picked a different module (perhaps because the core module wasn't ready?).  And in D7, Workflow has a large following.
There are also content moderation module which aims to define content states. 
From my understanding, core Workflows is API and UI to utilize the moderation states and transition. I am not sure if core Workflows could work with states machine, but logically it should.

Core Workflows and contributed core Workflow must be similar in their general goal, but provides different widgets, and architecture. 

It is very interesting question and I would be happy to hear some clarification.