What does this add/do?

Will be an addon for AdventureZ and adds a void dimension only for a boss fight against the Void Shadow.
The reason for it to be an addon is, the fabric api has no guarantee to update their teleport api when update 1.17 will come out and the chance is pretty high it will take a while to update it and AdventureZ should get updated quickly

So will this work on 1.16.5 if I use it right now?

probably not, it is still in development