Localsend has the option to share as a link (URL or QR code) that others can open in their browser, provided they are connected to the same local network.
There are a lot more people meeting this description who aren't serial killers than there are serial killers. So it must be some else that all the serial killers have in common that doesn't apply to the rest.
The national government is probably not able to do more than a very basic check on would be international travellers. Local government on the other hand is capable to scrutinize individual teachers much more thoroughly.
That warning is for people who e.g. select English but not Undetermined. If you select no language at all, the warning should not apply to you and you should see all posts.
Professor McGonagall is a Windows Vista user, apparently.