Another use case is that our Order Management System and our POS both need to add and remove tags from orders to help our shipping and CS staff in their workflows. Based on tags we have parts of the business segment and handle orders. We can add and remove tags via API, but if we cannot add comments via API, then we have no idea who (person or API) nor when a tag was added. So it makes it a nightmare for end users to know the order timeline of events. So now when we do the API updates to tags, we log them in our database, but then users have to ask Dev for details when researching issues or asking for timeline of tags.
Is this still not possible?
I don't believe it's available yet. Although you certainly have a good use case. While you could employ metafields to handle "logging" the product tag changes, these wouldn't be easily visible to the endusers in the web UI.
Just wanted to chime in that, in the year 2021, this would still be a great feature to add to the REST API.
As it stands, I am sending email confirmations to my customer service team as opposed to logging these changes in the order comments/timeline.
I also regularly get requests from store owners that want the timeline comments to trigger a webhook (integrating it with the order update webhook would make the most sense I think).
I know plenty of store owners that use the timeline as an essential part of their workflow, if we could have better access as developers we could help more store staff save time and provide a better experience.