In designing similar interfaces, I noticed that our users who were more familiar with physical documents preferred horizontal, while those more familiar with digital interfaces preferred vertical. Tiny sample size though.

If you look into the world of visual effects software, there's a lot of node graphs and it quickly becomes apparent where they fall into place.

Horizontal is for data flow based graphs where nodes pass specific data blocks to each other. This is the norm in shader editors, Maya's node graph, blender geometry nodes, Unity shader graph, Unreal blueprints etc...

Vertical is for pass the world or step based graphs. Each node represents a step, and nodes mutate a combined state that gets passed down. Nuke, Houdini and Katana are like this

the vertical graph has a lot of familiarity with people because it's like a Todo list of steps.

Horizontal graphs are better for data flow because they can have tons of labels that run vertically and get connected to each other.

In some cases you even have hybrid graphs. Where data processing runs horizontally that feed into steps which run vertically. You'll see this in Pixar's Presto, and Unity's Visual Effects graph.

But as the length of a written document goes toward infinity, whether the document is prose or code, it ends up going down.

And phones are portrait and web browsers all naturally want to scroll up and down.

The Git graph tools are vertical because terminals naturally scroll up and down.

So all my hand-drawn graphs are vertical, too. Top to bottom, just like code or very narrow prose.

For sure, what you're saying is true, but I suppose it depends on the content, layout, and how it's used. Vertical is typically better when first word type scanning is important, like data tables or anything similar. Horizontal scrolling in data tables is something I hate with a fiery passion and do everything I can to avoid, for example. When a workflow or series of interconnected elements are what's important than horizontal tends to work better for how people think about those things, imo, but there are exceptions to every rule. Basically, it just depends.

With a lot of marketing flow builders I’ve noticed that some if the older software has workflows that go right to left, whereas newer tools go from top to bottom.

I’m not sure if anyone’s testing the UX of these or whether designers are just following convention from other tools, however.

Do you mean "right to left" or "left to right"?

Left to right. I’ve not had my morning coffee yet, clearly

I prefer vertical as well. Find it much easier to follow, possible because I'm used to working with code, which is also laid out vertically (and where I tend to break and indent long lines to make them easier to read).