Assuming the default setup in LibreOffice, when users save some document to a non-ODF format they are warned about potential data loss. Nonetheless, they can change the default format at Tools > Options > Load/Save (General) and no warning is issued at that point, nor will be when they save their documents. I think we should also warn users about potential data loss in the moment they change the default format. Related: #90478.
Ok, let's set to NEW.
I consider to be normal, and not enhancement base of the fact that there could be data loss.
I consider it unnecessary. This isn't an area that someone would just stumble upon and randomly decide to make changes. Anyone reaching here is rather intentional about always saving in something that "normal" people can use. Plus I consider that warning to be pointless in the first place. It is more a marketing gimic than anything useful. Most users have no clue what that means anyway. (All my novice users wanted to know is what should I click to be able to continue on with my work.)
If implemented, it should honor the same "I know - don't ask me again" flag that already exists.
UX team: what do you think? Maybe I was too quick in 2015.
We discussed the topic in the design meeting. Saving with the default of plain txt/csv gives no warning but changing this on SaveAs to doc warns. Quite ridiculous. MSO gives a warning for odt (for example) even when it is the default (together with a don't show again checkbox). We should do the same and show a confirmation/message box (I'd do for _all_ formats except odt/fodt). The "[x] Show this warning" should be linked to the current checkbox in the options dialog that could be " [ ] Show a warning when saving in non-native formats".