FWIW, I 'whiteboard', flow chart and everything using google Sheets.

It's not pretty for presentations, but it works really well for engineers in many situations where the flow isn't super complex (think 2d-array instead of tree w/ many branches).

But if you want more tree like structures - there are a number of mind-map software that can change the representation to tree AND have keyboard navigation and control. Freemind being one.