Apologies up front but i gotta at least try...
Its a great shame this option isn't on the horizon or even being considered 
Dont get me wrong, I understand the sharing of flows issue & the visual identity stance but given the option to work like an SFC diagram or a top down sequence diagram over left to right scrolling - i'd bite your hand off & walk your dog for 2 months!
Is there no way we could convince you guys to at least give some thought to (if you did implement vertical flow) how you might be able to handle things e.g.
- mixed direction flows?
- auto reformatting?
- dual layout coordinates to permit vertical/horizontal switching of a flow?
- Import warning about flow orientation?
- Per flow tab option (so that shared works can be imported onto a TAB set for vertical flow)?
(i have more ideas haha)
You guys have done such amazing work on Node-red - Surely the differing visual difficulties could be mostly mitigated some how?
