In the "new" but not necessarily "better" model, a tray icon MUST have a popup menu and does not return, as separate events things like double click, drag, right click.
Desktops like Mate do still support the old, richer, system tray model but there is no guarantee that they will even the next release. So, you are well advised to design your apps to use only the new model.
(Sadly) most of your linux end users will be using Gnome (no, I don't know why) and it will not display the old model at all.
And, for the record, most newly released distros right now, on Gnome are using libayatana-appindicator3 rather than libappindicator3. Lazarus less than 2.2.* (or trunk) does not know about libayanta ! So, fail.
I am experimenting with a postinst script that creates a symlink, do not like it, but ...
EDIT: I should have mentioned one of the reasons why the old systray model will disappear is Wayland. Right now, its Gnome on Wayland, but the others are expected to follow.
Davo