I don’t need to be asked if I want to save a document every time I exit Writer. This may happen with other parts of LibreOffice. I’m too much of a newbie to know.
I understand your sense of annoyance. For myself, in my forgetful old age, I am sometimes glad to be reminded to save the document before exiting, particularly if I have made significant changes that were not yet caught by the last automatic save. So I tolerate the nag, knowing that it has “saved my skin” more than once.
Hack the source code and re-compile LibreOffice. No way otherwise.