Do these update when the website they are linking to changes? Or are they preserved in amber at the time of posting?
Linking to the wiki page for COVID as an example and to see if it changes…
The COVID-19 pandemic, also known as the coronavirus pandemic, is an ongoing pandemic of coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The disease was first identified in December 2019 in Wuhan, China, became a Public Health Emergency of International Concern in January 2020, and subsequently recognised as a pandemic in March 2020. As of 2 October 2020, more than 34.3 million cases have been reported worldwide, although the true…
The rendered HTML implies the onebox content was captured when you posted the OP and that preserved content will be served years later, even if the page in question changes later.
However, without digging in to fancier debugging than I’m willing to tackle, it’s notoriously hard to know for dead certain what was in the initial page load coming from Discourse vs what was populated in real time by Javascript on the page.
Have we not seen some broken links that once worked, where the preview box just says something like, ‘This content is not available?’ If so, then it’s rendered in real time by reloading the embedded content.
It could also be a hybrid between real-time fetching and static snapshots as well. For example, there could be a cache of fetched page snapshots on the server, but if stuff is invalidated in tge cache because of age or memory space it could be re-fetched when you load the page.