That date could be an Easter egg:
: Regular updates to the in-depth system that allows players to design unique character models for use in sandbox environments. Naughty Sandbox -2021-05-31- -Naughty Sandbox-
The 2021 builds of the original "Naughty Sandbox Core" were noted to be somewhat technically limited due to their status as a "first solo project," occasionally suffering from messy programming that hindered physical interaction features. The game is primarily hosted on and supported through That date could be an Easter egg: :