I firmly believe that if (as the SC suggested) configurable truthiness of a sentinel is too much complexity for this proposal then __bool__ should raise an exception. As has been previously stated quite a couple times now higher up in this thread, by making it raise an error by default it leaves us open to changing this later without backwards compatibility issues.
I’m also sympathetic to the need for custom truthiness, but I’d want to see that as a future discussion and decision separate from this proposal, to ensure it gets a reasonable amount of discussion on it’s implementation without impacting the completion of this proposal.