Plasma display technology was the rave back when Panasonic introduced it, years upon years ago, but then came the problems with the brightness dimming over time, susceptibility to screen burn-in and image retention, those sorts of things.
Most of these problems have been surpassed, but they came at a time when LCD also rose to prominence, which is why liquid crystal display is more popular among HDTVs now, and displays in general.
Between that and the energy saving and image quality advantages of LED backlighting, Panasonic has decided to lay off the plasma promotion.
That is to say, the company will keep manufacturing plasma displays, but won't push development any further.
Instead, it will try to see what it can do with OLED (organic light emitting diode).