After I installed 19.04 the icon was there. But after reboot it's gone and never appeared since (reboot doesn't help).
I couldn't make it back through properties of Notification Area and Indicator plugin, they're both on the panel.
ProblemType: Bug
DistroRelease: Ubuntu 19.04
Package: xfce4-panel 4.13.4-1ubuntu1
ProcVersionSignature: Ubuntu 5.0.0-13.14-generic 5.0.6
Uname: Linux 5.0.0-13-generic x86_64
ApportVersion: 2.20.10-0ubuntu27
Architecture: amd64
CurrentDesktop: XFCE
Date: Wed Apr 24 19:37:21 2019
ExecutablePath: /usr/bin/xfce4-panel
InstallationDate: Installed on 2019-04-22 (1 days ago)
InstallationMedia: Xubuntu 19.04 "Disco Dingo" - Release amd64 (20190416)
SourcePackage: xfce4-panel
UpgradeStatus: No upgrade log present (probably fresh install)
Xubuntu 19.04 is EOL. Marking this ticket as closed. If you are able to reproduce this issue in a current Xubuntu release (focal, impish, jammy), please reopen or create a new ticket. Thanks!