When I insert an URL in a post, the preview box shows this:
Sorry, we were unable to generate a preview for this web page, because the following oEmbed / OpenGraph tags could not be found: description, image
Example with others forum links like PunBB or PhpBB
https://forum.ubuntu-fr.org/viewtopic.php?id=2074792
https://forum.openoffice.org/fr/forum/viewtopic.php?t=66126
The replies are lacking <meta name="description" content="…"> and <meta property="og:image" content=…> so OneBox can’t preview. (So does https://documentfoundation.org actually, so unlike claimed previously that one can’t be previewed either.) Compare that with
or
or