Apologies for only replying now as it’s been a little extra hectic lately.
Some of the flags on the forum have been removed some weeks ago: they were not intended to be available/selectable, but the reason why requires some context.
The forum flags are a not native feature of this forum. But because we also had them on the old forum, when we moved to Discourse we found a 3rd-party plugin that does something similar which is what we have been using. Unfortunately, that plugin is not maintained anymore and is a bit quirky as it was designed for a particular forum where the plugin author needed some specific flags which probably made sense for his forum, but not necessarily others.
For instance, the plugin came with mixed country flags (US-GB, US-AT, DE-AT, etc) and other extra flags (e.g. Brittany). Importantly, several of those custom flags didn’t follow the 2-letter ISO codes which Postcrossing’s uses for Postcard IDs, URLs of pages, etc. I also fear that as the plugin is no longer being updated, on the long run it may actually become incompatible with future versions of Discourse and there are already some flags in the plugin that are outdated.
Ideally, we would like the forum to simply use the main website flags (automatically), but I simply don’t have the bandwidth to code that up at the moment. However, updating the plugin to drop the extra flags that don’t follow the 2-letter codes is somewhat straightforward, which is why some of the flags have been dropped to be closer to what Postcrossing’s uses.
I understand that those were using the flags that were removed are now missing them and in retrospect I should have written about this earlier. I see also that those that had selected one of the removed flags were showing a broken image which I have now fixed. Some seem to be using the status feature to display custom flags which is a neat workaround. I hope in the future the flags will simply follow Postcrossing’s account automatically (like username, country, email, etc) as to minimize these issues.