I'm not sure when it was disappeared, but it is disappeared forever, I just wonder why and/or what package(s) update canceled this feature on whose function many applications based.

For example, KeePass2, GoldenDict, Slack, etc have a feature minimized or closed to system-tray icon, while left-bottom system-tray disappeared, those feature non-function too, center-top message tray just haven't any function at those situations.