The legacy tray introduced as part of the notification redesign in
3.16 was meant as a stop-gap solution to encourage applications to
move away from the concept of status icons, but it hasn't really
done anything except of getting in the way. Given that the large
majority of apps that still make use of status icons work perfectly
fine without them, we decided that it is time to drop this unloved
bit of UI altogether. Users who still want them (or use one of the
odd cases where an app really depends on the icon) can install one of
various extensions that are available, either based on the XEmbed
support that is still kept around or implementing the DBus-based
StatusNotifier spec.

https://bugzilla.gnome.org/show_bug.cgi?id=785956