Now that the Winter release is out and Flows can send outbound messages will the test be updated or is the answer still a workflow?

Yes, now you can use this feature from Flow as well as per the recent winter 22 release.

Refer:

https://help.salesforce.com/s/articleView?id=release-notes.rn_automate_flow_builder_outbound_message.htm&type=5&release=234

still you can use & works with workflow, but as per salesforce roadmap they planning to retirement of Workflow Rules and Process Builder later next year.

https://admin.salesforce.com/blog/2021/go-with-the-flow-whats-happening-with-workflow-rules-and-process-builder

so your answer in shorts - yes its still work with workflow, but if you like to create a new outbound message then try using flow instead of workflow.