The 3DS is slowly fading from memory since its 2011 debut. With a lack of presence at E3, the elimination of 3DS My Nintendo rewards in Japan, and now the removal of the YouTube app (also in Japan), the system is being phased out, despite what Nintendo says.
According to Japanese Nintendo, the ill-fated day of YouTube's death knell has arrived, and the app was removed from the eShop and support has ceased. We knew this was coming: Nintendo warned us it was happening, but it's sad to see a piece of an online service go regardless of the context.
To be clear, this is mostly symbolic in nature. The 3DS YouTube app was not a great way to watch video in any fashion, and although the handheld video angle was more of a draw back in 2011 (or before), the ubiquity of smartphones has made them less alluring for multimedia use.
Although this may seem insignificant, remember this day: whenever Nintendo decides to discontinue the 3DS, it'll follow an innocuous announcement like this.