There has recently been some confusion regarding how external algorithms are considered for inclusion into Stan. Over the past few years the core algorithm developers have adopted consistent criteria for consideration that we have individually shared with those who have proposed new algorithms and methods. Unfortunately these criteria were not made publicly available and consequently not everyone was aware of those criteria. This ultimately led to varying expectations and disagreements.
In order to avoid such confusion in the future, I have formalized a policy as the current Algorithm Tech Lead, with input from the active developers who have most closely worked with new algorithms and methodologies. The policy is available on the Stan Wiki, https://github.com/stan-dev/stan/wiki/Proposing-Algorithms-for-Inclusion-Into-Stan, and I encourage you to send anyone inquiring about new methods there so that they are aware of what will be required of any new method.