As part of every GNOME release (spanning GNOME Shell, Mutter, core apps, etc) is an official set of GNOME Shell extensions. This is non-default package is
AppIndicator more or less allows an application to send an icon and a list of menu items to the desktop environment to be displayed there in some way. Classic tray icons, on the other hand, are actually part of the application that provides them, the system just puts them in a common row for all applications.
While this type of tray icon is more flexible because applications can do anything they want there, AppIndicators are a better/safer solution because … well, applications just cannot mess around in the system tray as much.
Personally, I thought the classic tray icons were as good as dead (on Linux), but I guess GNOME can always surprise. Not only in removing stuff but also in bringing it back.
Actual legacy tray icons.
AppIndicator more or less allows an application to send an icon and a list of menu items to the desktop environment to be displayed there in some way. Classic tray icons, on the other hand, are actually part of the application that provides them, the system just puts them in a common row for all applications.
While this type of tray icon is more flexible because applications can do anything they want there, AppIndicators are a better/safer solution because … well, applications just cannot mess around in the system tray as much.
Personally, I thought the classic tray icons were as good as dead (on Linux), but I guess GNOME can always surprise. Not only in removing stuff but also in bringing it back.
Ah, that was the explanation that I was looking for. Much obliged.