Hi all, sorry for the delayed reply, my spouse and I were a bit under the weather and have some catching up to do.
@TaylorItaly, yes, code to unlock Tier 6 was added as we started looking at new tenets for this tier, and it was accidentally left in when we shared the Tier 5 tenet update. We will upload a version of the Tier 5 update that doesn't have a Tier 6 unlock with no Tier 6 tenets. Thank you for pointing this out.
Re: Monotheism. Obviously, the simplest route to including Monotheism would be to add it as a choice at the point when players currently can select Polytheism or Shamanism. However, as pointed out, this could be construed as jarringly ahistorical in a game that already has some fairly jarringly ahistorical elements. Further, doing so would cause religion to work differently than other HK systems work (such as the military and infrastructure systems) which tend to flow in a fairly historical fashion (i.e., you can't develop crossbows or grain silos in the Ancient era). We're still reading more about the development of religion and thinking about how best to incorporate its various strands while we plan the 6th tier tenets. At this point, it isn't even clear whether Polytheism and Shamanism should appear at the same time (there is at least some evidence that Shamanism significantly predated Polytheism).