The Mario Kart TV website will cease to exist early next month.
The in-game service itself will remain functional, so you can still upload to and watch Mario Kart TV from within Mario Kart 8. In addition, videos uploaded to YouTube will not be deleted.
Nintendo announced this through a notice on the site itself, and thanked fans for their support.
The site goes offline at 11pm PT, April 4 and 7am UK on April 5.
Thanks, Nintendo Everything.