i don’t personally have anything in writing, but i had lunch with the notion founders in 2019 and they mentioned they got rejected by YC when notion was “more of a research tool”
the canonical YC advice is to make something that you want, then to go find people like yourself and see if they want it too. whether that turns into a real market or not has a lot of luck, but it's often surprising how promising being your own user can be, especially when you solve a hair-on-fire problem for yourself!