With the new Export Service | ArcGIS REST APIs | Esri Developer we can always recreate a SD file from an existing service, so this functionality should be added to the service item to the "Export Data..." button and support it with the "Update Data" button.
There could be an optional setting during publishing to keep a copy of the data when publishing using "Copy Data" as "backup", stored directly with the Portal item. But this should not be the default. SD Files using "Copy Data" increase the size unnecessarily in most Portal deployments I know.
Once an SD file is created, there should be a way to see all the properties embedded in it.
SD Files should not be listed as separate items Re: Why are Service Definitions Separate Items? - Esri Community
This ArcGIS Pro idea is closely linked to get more benefits from the generated SD files: Be able to Import Service Definition (.sd) file in... - Esri Community
This idea is also proposed for ArcGIS Enterprise.
