Comment 2 for bug 1769438

Since at least Trusty, lxpanel-indicator-applet-plugin (which does not appear to be provided upstream), suggests indicator-messages-gtk2, which is not available. It also suggests indicator-application-gtk2 which has a hard requirement for gtk2 while indicator-application is gtk3. It seems that the goal was to split out the gtk2/3 versions. However, it doesn't seem that indicator-messages (a package which is available) has a hard requirement for gtk at all, so they may have just dropped indicator-messages-gtk2.

That said, I think the solution here is to replace indicator-messages-gtk2 with indicator-messages but it will need some testing.