In order to undo/redo or save/discard feature service changes during editing in ArcGIS Pro, the feature service must be registered as branch versioned. Versioning is not possible in a file geodatabase. Many of Esri's users work for smaller organizations without the need, or budget, for an enterprise geodatabase setup.
The workflow in ArcMap: add a feature service from AGO, create a local copy for editing, edit the feature service (allows undo/redo and save/discard changes), and then publish edits to the server.
The workflow in Pro: add a feature service from AGO and edit the feature service (no more local copy needed, but all edits are final).
The safe workflow in Pro: add a feature service from AGO, Data > Export Features, edit the new feature class, overwrite the feature service with the new feature class (Overwrite Web Layer).
Note: The workflow in ArcMap and the safe workflow in Pro are further slowed down, as creating a local copy or exporting the feature service also means downloading all of the attachments associated with those features (and then uploading them all again).
The proposed workflow in Pro: add a feature service from AGO and edit the feature service (allows undo/redo and save/discard changes, doesn't download all attachments).
Plenty of Pro/AGO users won't have access to an enterprise geodatabase to utilize branch versioning. It would be nice to be able to add a feature service from AGO and immediately edit it (with full editing capabilities) without needing to follow a time consuming workflow or paying for a service we don't need.
This has been marked by Kory Kramer as a Duplicate of https://community.esri.com/ideas/15490-undo-feature-service-edits-in-arcgis-pro.