This feature (Revision History for Contact Workflows) is currently in private beta! This feature will be available as part of a broader under-the-hood renovation: moving contact workflows to our new workflows system.
Fill out this form to request earliest available access for your account, which will happen when we begin public beta, over the course of the next few weeks: https://forms.gle/oUkexNjHvqoKV4Bt7
When access is given, your most recent contact workflows (those created after August 5th) will be moved to the new workflows system, which gives access to this feature and more. To move an older contact workflow to the new system, you can do so by cloning it: every newly created workflow in your account will then live in the new system.
Please note that once you are opted into the new system, you cannot be opted back out. Read about all of the changes the new workflows system will bring, here: A New Engine for your Workflows
Again, this update is for workflow revisions only, and you can add your usecases and upvotes to the below linked forms idea, if you would like to continue the conversation around this functionality for the forms tool: