Is there a way to undelete an O365 workflow?
No, If you have deleted a workflow (Or a list/library with workflow ), then the workflow can't be recovered.
Maybe you could ask MS if there is a chance to restore the workflow from any previous backup there made. But that is only an idea, I didn't tried out before.
This is upsetting, I just deleted one because it showed up in "SharePoint Workflows" and I was like "What, no" and deleted it. Dumb of me. Thought path was, "Oh well, it'll be in the Recycle Bin". Nope. Nope it is not.
Why was it even showing in SharePoint workflows ....
Retrieving data ...