A Best Current Practices document for operators of DNS privacy services had been developed in the IETF. (Thanks to the Open Technology Fund for supporting the work on this by dnspivacy.org)
The document is available here: BCP232
It includes sections on:
Some of the research that was done as background for this draft is also published here:
Also see the work done by the EDDI in developing the European Resolver Policy which is heavily based on BCP 232.