My answer is based on Mik's** very thorough response here: What is dconf, what is its function, and how do I use it?.
I'm assuming that your Xubuntu is a default one and that you are using Network Manager with nm-applet.
To reverse the action of clicking "Never show this notification again", open your terminal and paste in the following:
gsettings get org.gnome.nm-applet disable-vpn-notifications
The status will be returned to you. In my case, the response is "false".
In your case, to revert to seeing the notifications again, now paste in the following:
gsettings set org.gnome.nm-applet disable-vpn-notifications false
Alternatively, you could just reset the value to the default by using:
gsettings reset org.gnome.nm-applet disable-vpn-notifications
You may need to disable networking and renable it or log off and log in again or reboot for the changes to take effect.
Edit: there is a GUI if you want to avoid using the terminal. It's called dconf editor and you can get it from the software center or by running:
sudo apt-get install dconf-tools
Again, Mik's answer describes its usage but, at least in the past, there have been instances of confusion: an example is here: Are dconf schema names case-sensitive?.
**Mik has asked for his account to be deleted according to this.
Delete Indicators plugin, install xfce4-pulseaudio-plugin with command
sudo apt install xfce4-pulseaudio-plugin
and add it to the panel. You will have your volume controls without messing with the buggy Indicators plugin.
There is also an alternative version of Indicators plugin which uses GTK2 and has a classic tray behavior, it might be free of bugs.
Best Answer
Just reinstall the Xubuntu icons from the repositories and that will get your icons back.
I found this out by querying each of the icon folders with
dpkg -S; for example:which returned the name of the package they came from:
If you have any other problems regarding settings you can purge the
xfce4-indicator-pluginand then reinstall it withIf you have also removed the indicator from the Xfce panel, add it back again by right clicking the panel and clicking panel > add new items (see screenshot). Importantly, you will probably have to logout and login for the changes to take effect after reinstalling the icons.