Update: As mentioned in the first comment, this was an April Fools prank. Read this for details on Plasma Experiences.
The mock-up dialog would allow the user to choose from four different looks for Plasma: the modern Plasma desktop, Plasma Desktop 4.11, KDE 3.5 and KDE 2. The choice dialog would be presented to the user after the installation of KDE-based distributions. Choosing one type of interface would affect the Plasma theme, plasmoids, widget theme, window decoration and all the other KDE visual aspects.
According to Thomas Pfeiffer,
The chosen experience would affect the Plasma theme, the selected Plasmoids, Panel config, widget theme, window decoration and everything else that makes up the desktop experience, even down to the available features! In case of popular demand, we might even integrate KDE 1 support, though it would pain me to have to put five screenshots in that dialog ;)
Currently it’s just a proposed idea, but things may start moving on, although development wouldn’t be easy work.
Plasma Next is the upcoming KDE project, scheduled for release in June.