I want my GTK application to have a status icon in the system tray, I have got this working quite nicely using a StatusIcon, however this has been deprecated:
gtk_status_icon_new has been deprecated since version 3.14 and should not be used in newly-written code.
Use notifications
The documentation says "Use notifications" but I have no idea what this actually means. I interpret this to mean "use desktop notifications", but as far as I can tell that serves an entirely different purpose. I want an always-visible system tray icon, not a temporary popup notification. Despite that, I can't find anything else which would seem to be what is meant by "Notifications".
Does there exist a new replacement for GtkStatusIcon which provides an icon in the system tray, with support for receiving mouse click/scroll signals and displaying popup menus?
Unfortunately yes, they refer to desktop notifications. If you still want to use system tray icons, then keep using GtkStatusIcon; just note that it is likely going to be removed soon. (The point is that more things have desktop notifications than status icons, but I'm sure there's other reasons as well. GTK+ probably shouldn't deal with desktop environment-specific stuff like that anyway... There's nothing stopping anyone from making a GtkStatusIcon replacement of their own, but no one seems to have done so yet.)