I just tested this on the latest desktop version of Excel on Windows 10, and I did not get the 'large amounts of data' warning with only a single cell copied in the clipboard (the cell contained the word 'foo'). Increasing the number of cells copied to a large number did cause the warning to appear. I did not investigate further to see exactly how many cells need to be copied to cause the warning to appear.

> in Excel's case that question "Do you really want to quit? You have copied large amounts of data to the clipboard." even occurs when you have a single cell's worth in the clipboard

That's not correct. You need to have quite a few cells in the clipboard for this to happen.

> But that's also a confirmation question that no one really understands

The dialog tells you exactly what it is asking for. It gives you three options: keep the app open, store the data in the clipboard, discard the data from the clipboard. I am not sure how a user could not understand what to do in that situation.