Is there an official limit on the depth of a nested create mutation? I think I've hit a limit at 4 nested levels (error when running the mutation and the generated API documentation seems to say there is no array field for the relationship on the generated type at this depth)

that's exactly the current limit

Thanks. Are there any plans to increase this? (I understand that the required types get unwieldy as you add more levels of potential nesting...)

could you please create a new feature request: https://github.com/graphcool/feature-requests? Also, I'll add this limitation to our docs 🙂