I don't think a to-do app that could effectively help ADHDers ever existed. Never heard of one, probably never will. If such a tool exists you will def see it in r/ADHD or similar communities. When you have no competitor in a market probably the problem is the market. I don't think a niche market for ADHD todo apps exists. Because the problem is not solvable by a todo app. Just like you can't effectively help austic people by offering social manner courses.