They state it clearly in the docs that Append is only an option in AGOL. They don't state why this is the case, however, and that has always bothered me. If you're going to hold back one of the more useful tools for dealing with hosted feature layers, I'd like to have a reason. And if it's a legit reason, I can live with it, but that's preferrable to not knowing.
Having dealt with this in the past, the only "easy" way of dealing with this situation is just to overwrite the hosted layer with the updated spreadsheet. As long as the user hasn't removed anything from the spreadsheet, or you haven't made significant changes to the data since adding it, you should be good.
If there have been changes or deletions since publishing the layer, you'll need some way of comparing the two datasets separately before submitting new features to the service. You can do this programmatically via python (we use pandas' compare function to do just this sort of thing), or you can bring in the CSV to Pro and copy/paste features, but that requires more manual looking, filtering, possibly spatial selections, and can be arduous if there are lots of records, and is prone to error.
- Josh Carlson
Kendall County GIS