Can I use it?
Yes! There are currently multiple distributions shipping Plasma Bigscreen.
The interface is using KWin over Wayland and is now mostly stable, albeit a little rough around the edges in some areas. A subset of the normal KDE Plasma features are available. This makes it possible to use and develop for Plasma Bigscreen on your desktop/laptop.
What can it do?
There are quite a few TV-optimized apps that are now being bundled with the various distributions, allowing a wide range of basic functions. These are mostly built with Kirigami, KDE’s interface framework allowing convergent UIs that work very well in a TV remote-only environment.
Where can I find…
The code for the shell can be found on invent.kde.org.
You can also ask your questions in the Plasma Bigscreen community groups and channels.