None of those are supposed to stay on the background (specially not browsers, which are sandboxes for untrusted apps nowadays).
But if I click X on my email client, it keeps receiving email; if I click X on my torrent app or JDownloader, they keep downloading; if I click X on Steam, it keeps updating my games; and if I click X on a chat application, it keeps receiving messages and, yes, usually keeps voice chat open.
And BTW, browser tabs sadly do stay open in a twisted way: notifications, often malicious for the kind of users that accept the prompts without reading.
In this case, Discord is still transmitting data after you press "X".
It is the wanted behaviour, of course, but I don't find it weird that having a message explaining it the first time you close the window should be mandatory.
But if I click X on my email client, it keeps receiving email; if I click X on my torrent app or JDownloader, they keep downloading; if I click X on Steam, it keeps updating my games; and if I click X on a chat application, it keeps receiving messages and, yes, usually keeps voice chat open.
And BTW, browser tabs sadly do stay open in a twisted way: notifications, often malicious for the kind of users that accept the prompts without reading.