Workflow should keep history for each workflow notes during as well as after completion of workflow.
The Admin user should see all workflow steps with notes at any time.

 

The standard use-case of using workflow notes is:
Create 3 steps workflow for content type with workflow notes enabled
Execute the workflow by adding notes for each stage, Review, Approve, Publish.
At the stage of approval the user can see only the note from the previous state.

For example:
If user A sends content for Review and adds a note "I have reviewed it its OK"
If user B reviews the content from user A and approves the Review and sends it for Approval with Note "I approve this".
Then user C will see the note from user B "I approve this", but not the new from user A.
 

There is no functionality to show the notes from the different workflow states. The different states e.g. SendForReview, SendForApproval, SendForPublishing do carry individual notes, the notes are valid only for passing from state SendForReview to the next state, etc. so a code customization can be implemented.

Notes from the different states of the workflow are saved, the data is persisted in table [sf_approval_tracking_record] and can be used if needed for customizations. To achieve the goal of showing all notes at the final state of workflow or moving the notes from one state to the next has to be done trough a code customization.

The customization has to intercept the saving of any type of content in Sitefinity and be able to execute Sitefinity API once intercepted. Such a interception point is IDataEvent Sitefinity Documentation, Idataevent https://www.progress.com/documentation/sitefinity-cms/for-developers-idataevent
it will execute upon any action a user does and allow for code executions to occur.

Sample of an actual interception in How to catch the Publish, Unpublish and Save as draft events using API 

The next thing is to have access to modify the workflow notes which as I mentioned above are saved in table [sf_approval_tracking_record] in column [note] and there is a separate [note] for each state of workflow approval.
The API for working with the data in table [sf_approval_tracking_record] including the [note] is sampled here.

The API queries the data in table [sf_approval_tracking_record] once the developer has access to a single content in Sitefinity. Lets take for example a News item in Sitefinity.
Once a newsItem is obtained and there is a reference to "using Telerik.Sitefinity" the method .GetApprovalRecords() can be used on the news item.

newsItem.GetApprovalRecords();

This gives a list of all entries form table [sf_approval_tracking_record] for this news item. If there are workflow notes written by users they will be available for modification.
At the last state of approval and publishing the code in IDataEvent can execute and copy all notes from the previous workflow states to the Note created for the state of publishing the News item and this will be the summary of publishing this news item.