How much of this shape is an artifact of how google is presenting the data? The right most categories are chunked in ever increasing increments.

Yeah, I tried to address that in the end of the post. You'll notice links to two other site's data where the chunking is present but not nearly enough to make a horseshoe.

Without Google's chunking it would look different. but it actually makes it easier to see what's happening. Everyone will see a 1-log like function here (without chunking), what matters is the rate of decay, and that is well-captured in the chunking.