> It's a special option you have to select, but when done properly […]

So not really reliable, I wouldn't trust ordinary office users to use it correctly. Exporting to CSV is safer, because you can open the file in a plain-text editor and see what you're actually going to share. CSV won't hide anything from you, unlike the proprietary Excel file format (I know it's XML-in-ZIP but who can be bothered inspecting the contents).

Why is it not reliable? The much famed "ordinary" user is often portrayed a bumbling incompetent user. For any heavy excel user, or rather any excel user thats ever used formulas, this is a common UI operation. Your CSV suggestion is good, but it can't carry the formatting and other extra info which is the reason people use excel to begin with!

No it's not because you never know what other data is quietly hidden somewhere else in that monstrous opaque file. MS Word used to silently include deleted text to help it save/open files faster! You really need to look at all the bytes in the file to be sure, which means export to CSV, or I suppose unzip the XLSX file and hope there's no binary data. The same goes for metadata in images, and any other complicated opaque file format. It's automatically potentially full of things you don't know about unless you're an expert on that particular format and software, which no government Excel user is.

Another good way would be to export the data as a CSV file and then double check that before releasing it.