NES Golf was found in the Switch firmware, and rumors have now surfaced that it was done as a tribute to Satoru Iwata.
So why NES Golf? Well, Iwata programmed this particular game during his time at HAL Laboratory, so it is of particular significance in that respect. According to Justin Epperson (thanks, Eurogamer) - a producer at localisation firm 8-4 - NES Golf could be seen as an "omamori" - a charm that is usually placed at shrines in Japan and offers spiritual protection.
Apparently, on July 11th (Iwata's death day), if you do his "let's take a look" hand gesture with the joycons it will unlock the game. You can not change the Switch clock once you update the firmware though. So you need an out of box Switch, or to wait for next July 11th to try yourself. Still, a neat little gesture to a titan of the industry.