While every effort in KDE Telepathy went to the Qt5/KDE Frameworks 5 porting and KAccounts migration, there are still distros/users that have KDE Telepathy 0.9 (the kde4 one) running in Plasma 5 session. All the applications of KDE Telepathy work just fine, the only problem are the Plasma applets, those are totally incompatible.
So for those cases I've created a small Plasma 5 applet which works with KDE Telepathy 0.9. It can bring your accounts online or offline or open the contact list and accounts settings, just as you're used to and the only dependency is TelepathyQt5. However because of changes in Plasma 5 stuff, it's not perfect.
Every applet in Plasma's systray now must have a full popup-on-click by design. This is in direct contrast with Plasma from kde4 times where applet could be just an icon. This hinders the effect of bringing the contact list up upon simply clicking the applet; now it will open a popup with a button inside. There are two solutions to this though. As middle-mouse button click can still be handled however we want, I've added opening the contact list window to middle-mouse button click too. Or you can just put the applet on the panel, there it will work just fine and you can't tell the difference.
I've also considered having the icon simply be a KStatusNotifierItem, but that brings more problems than it solves, namely something needs to create and manage it, which would mean new process or new kded4 module and all this complexity was not really worth it, given this is supposed to be a sort of short-time transition crutch only. But other than that small issue, everything works just the same as its newer brother.
The new applet from 15.04 release then features a quick contact list in the popup with a search field so you don't even need to open the full contact list, but you still can either by middle-mouse clicking or from the context menu.
I don't intend to work on the legacy applet anymore, but if you wish to extend it, you can either get it from kde:scratch/mklapetek/ktp-legacy-presence-applet.git or download the tarball from KDE mirrors (there will be version 1.1 released the same date as KDE Applications 15.04).