Users often work with confidential material, which content should not bee seeable for other people. An example is when you want to show a costumer a document, want to open this via StartCenter, s/he could see content not intended for him/her. Setting a password is one, but not always a good solution. As creator you probably want to see in which document which information are as an overview/thumbnail. A possible solution could be to create a checkbox in top of the Startcenter to choose the visibility of the below contents.
I think that there's probably an advanced setting to do what you asked.
"An advanced setting" is surely a bit too much for an "housewife" like me or other users. ;-) Need just a simple checkbox or some such to hide/show documents content (depending on password protection, of course) ad hoc. A tooltip may warn about potential risks about this feature, when exist.
Simple enough... Tools -> Options -> Advanced: "Open Expert Configuration" enter "RecentDocsThumbnail" into the Search field, and search click the "edit" button to toggle True/False OK out exit & restart LibreOffice. On restart all Thumbnail Views have been replaced by generic ODF icons. *** This bug has been marked as a duplicate of bug 74834 ***