Glad to see you're looking at the API explorer. Just a guess here, that you saw the workflow template in the proof approvals object.
1) Go back to the document version object, and switch your API explorer version dropdown to unsupported.
2) check out the references tab and collections tab. You'll find that the proof approvals are in the collections tab. This means you can use syntax like this article:
https://experienceleague.adobe.com/en/docs/workfront/using/reporting/reports/text-mode/reference-col...
3) From your collections tab, you know your collection is "proofApprovals".
4) Your valuefield or valueexpression would be the workflowTemplate field.
Pointing out the obvious pitfall of using a collection here, which is that if you have multiple proof approvals, each one will contain its own Workflow template. There's therefore no way to have one listing that tells you what workflow template was used, unless you have a common name or a unique name across all of them. i.e. a valueexpression that either says "if the proof approver is john smith, then the workflow template is____" (and john is in all of them), or conversely, if the proof approver is one of 6 different people (who don't overlap on any of your templates), then the workflow template is ____.