I've found lots of examples of outstanding physical performance under a vegan diet, but I've been unable to find examples of bold theoretical breakthroughs being made under a vegan diet (the closest example I could find was Ramanujan, but there were other things about Ramanujan that suggest that there's absolutely no way most of us could live the same life and then end up in the same place), and my own experience has been really discouraging. After about 10 days on a vegan diet, regardless of my energy levels or legible performance metrics, I'll pretty reliably stop being able to, or stop being interested in progressing original ideas.
There are lots of possible exits here: Vegans and inventors were rare until very recent history, we shouldn't expect to have many records of people who were both, we might end up with none, even if there's no relationship between those things. It's also fairly likely that the effect would just be a result of a creatine deficiency, or a choline deficiency, or something like that, which can be fixed with a supplement (although Dr Gregger doesn't recommend it due to the incidence rate of high contamination in creatine supplements, but maybe you can find a brand you trust.)
(Anecdote on choline: I have a vegetarian friend who, at some point, stopped thinking in the sort of focused/precise/fluid/driven way that our project needed. They speculated that it might have been because they were on choline initially, then stopped. So I said yeah, I noticed a change, try getting back onto the choline? I don't think they ever got around to it. The lack of concern that they showed is actually one of the things that bugs me about this, because I also experience that, when I'm low: It kinda seems like we have be on the supplement to hold onto an understanding of why we need to stay on the supplement, and as soon as we lapse we forget what it was like, and how important it was? (Depression also seems to work this way. A depressed person often cannot imagine or remember not being depressed.))
There does seem to be a consistently replicated finding that supplementing creatine enhances memory and intelligence in vegans. (and doesn't for omnivores) (baseline cognitive performance in vegans is similar to omnivores, but personally I'm kind of expecting it to turn out that to be a result of vegan-leaning demographics starting on a higher base, then being lowered)
But even if it can be fixed with a supplement, I don't think most of us are taking creatine and choline! We probably need to have a conversation about that.
Previous discussion of health and veganism, when they touched on concerns about cognitive impacts, generally failed to allay them.
It wasn't my prior at all. It was a response to observations that took many years to arrive at, it is also a response to some empirical evidence, which I mentioned.