Is the rstan3 directory still needed?

[jeffreypullin]

Summary:

The rstan3 directory has not been altered in 4 years and therefore seems abandoned. Is it necessary to retain it in the repo?

Description:

My understanding is that rstan3 was to be a new interface for rstan, however it now seems to abandoned. If it is truly abandoned would it make sense to remove it from the repository? This would have no functional change but would make the repo simpler and therefore easier to understand for new developers.