2016 is a private client of Finobe.
Before 11:54 AM EST on December 20th, 2018, it was only available to members of Nobelium. As of this time, it is not yet available to the general public, but is planned to become so in the near future.
The 2016 Finobe client was first added in mid-2018. At this time, it could only be used by members of Nobelium, the now-inactive parent of Finobe, which itself is a publication of Nobelium.
This remained the case until 11:54 AM EST on December 20th, 2018. At this time, Raymonf (owner of Finobe & Nobelium) posted the following to the #announcements channel of the Finobe Discord server:
"trusted now has 2016, public release soon if that goes well
not that exact version though" --Raymonf, December 20th, 2018
This meant that users of the Finobe Discord server with the Trusted role were being given early access to the 2016 client, and that all other users may receive access later on.
He later followed this up with another post at 12:02 AM:
"note that the only reason we're releasing it is because roblox removed non-fe/experimental mode games
so while FE does completely work, you can still disable it" --Raymonf, December 20th, 2018
This was referring to Roblox's decision in mid-2018 to force all games to use the FilteringEnabled option by default with no option to switch back, therefore rendering Experimental Mode unused.
This also means that the client is being released simply to allow users to disable FilteringEnabled on their places.
We don't yet have a set public release date for the 2016 client, and in traditional Finobe fashion, we likely won't have any advance notice before this change is made.