The option is: 'Always warn if the document is to be saved in a format other than ODF.' I had deactivated this option, because when working on MS Word documents, the warning is unnecessary. However, I just had a traumatic experience with a document which had TXT extension when I first loaded it, which I spent three hours on formating with Writer and then pressed ALT F4. It was duly saved in TXT format ... Of course, my fault. However, it would make sense to modify the said option. It could read, instead: 'Always warn if saving the document in its own format entails loss of formating.' This would require Writer to check whether formating would be preserved in the format chosen by the user. This, however, is a feature that Writer already has. Even now, if I choose to save a document in a legacy format, a dialog box pops up telling me that some formating of the document may not be susceptible of saving in the chosen format. If such a box had popped up with my TXT file, it would have saved me three hours. Thus, the suggestion is to render a function that LO Writer has, available as an option.
WONTFIX in my opinion because of too many dependencies for the various file formats. You deliberately disabled a safety function and want now a secondary functionality depending on what happened while editing. Given that's possible easily (what I doubt) the user may get accustomed to it and expect the same for other relations. Meaning for rich text you get a warning that fields will not be saved, on html that RTL direction may not work in browser foo, etc. Pandora's box. But feel free to reopen if you disagree.
No, I do not "want now a secondary functionality". As I said, the function that I suggest making available already exists: You have an open ODF document, > do "Save under" > in the "Save" panel at the bottom, where it says "All formats", choose a file format other than Writer, > do "Save", > you get the message: "Dieses Dokument kann Formatierungen oder Inhalte enthalten, die nicht im gegenwärtig gewählten Dateiformat „...“ gespeichert werden können." (I will translate it if needed.) This is just the function I meant. Why not make it available at one more place?
Sorry; apparently I got confused. The message I mentioned > only pops up if the option "warn me" is activated > apparently pops up for every text format which is not ODT. So it appears that actually no check on loss of information is performed. So we leave the issue closed. Sorry for the trouble.