OK so I dug into it:
UDAFResolver2 Marked as deprecated and added GenericUDAFResolver2 & AbstractGenericUDAFResolver
HIVE-287. Support count(*) and count distinct on multiple columns …
John Sichi committed on Jul 11, 2010
Marked as deprecated --> GenericUDAFResolver2 & AbstractGenericUDAFResolver
HIVE-17132: Add InterfaceAudience and InterfaceStability annotations …
…for UDF APIs (Sahil Takiar, reviewed by Ashutosh Chauhan)
master
rel/storage-release-2.8.1
…
rel/release-3.0.0
@sahilTakiar
sahilTakiar committed on Aug 22, 2017
From the notes (HIVE-17132) it's not actually clear that UDAFs where intentionally added and included. In fact I might suggest that there was a well intended mistake when UDAFs were added and Resolver2 was depreciated. It seems this change wasn't really reviewed in depth. (I really can't be sure that there was intent to add UDAFS to this feature as there is no language that speaks to it. It seems to be accidentally lumped in with UDFs and this is where I think there was a mistake. I think it might have slipped through the cracks.)
Definitely there is a bug that it's not well documented what should replace resolver2. Please take a minute to read through the links and form your own opinion. It is worth sending an email to the dev list for hive and seeing what you get back from it.
I would continue to use AbstractGenericUDAFResolver.
