It is the one you keep finding scattered all over the Web. Here is another case.
The LibreOffice writer tips by Make Tech Easier include this gem as FIRST tip:
“1. Change the Default Save Format to Word”, because:
“The ODT format that Writer saves documents in by default is a nice idea in principle… But the reality is it’s not perfect, and if you’re sending an .odt document to Microsoft Office users, then they may run into a few hiccups opening it. For that reason, I like my default save format to be the more universally-compatible Word (.doc) format.”
What is really universal here is not the “compatibility” of the .doc format, which isn’t assured among different versions of Microsoft Word. It is the passive, every year less tolerable acceptance of the flaws of Microsoft Office (more exactly, what is harder and harder to accept is not Microsoft’s behaviour, but the passiveness of its users). In 2018, setting the default format in any office suite to .doc, .docx and the likes is terrible advice for all the reasons already explained below. Do NOT fall into that trap. Please.