The only way I can think of to do this is to allow the user that would run the sync into Prod to only have access to Test and Prod. Otherwise, DEV would also always be compatible. If the Workspace admin only has access to TEST and PROD then they would not be able to access DEV, nor its revision tags. That might be a weird concept for user access, but it is the only way that I can think of!
If all the testing is complete and user has given the green signal to deploy, Copy the TEST to PROD workspace(If it exists) or copy TEST to PROD in same Workspace and rename as PROD.(May be you need to delete the data in PROD after deployment - depends on the project)
Make a small change in DEV and create a revision tag.
Move the Changes to TEST from DEV.
Next Move the changes from TEST to PROD model and you are all set.
However If your client wants it to do that way. Its our responsibility to make them understand what are the best practices and the need of having different workspaces. Have you ever considered splitting the same workspace into two. Splitting workspace is not a big ask you can raise a ticket with anaplan and they will be able to split the workspace without having to add/buy more space.