From arc help land, --onto is said to be related to the arc.land.onto configuration.
On the other hand, from arc get-config --verbose, arc.land.onto.default is said to be the name of the default branch to land changes onto when arc land is run.
Is arc.land.onto.default deprecated? Why is it still referenced in ArcanistDiffWorkflow.php?
That's an interesting question.
Using git grep, it looks like arc.land.onto.default is only ever used in arc diff flow (to pick the default branch to diff against), and arc land is using arc.land.onto.
However, arc get-config doesn't mention arc.land.onto at all - that might be a bug in arc get-config, because arc.land.onto is set up in an ArcanistConfigurationEngineExtension, and is (presumibly) available for use in arc land.
It might make sense to try to dig into the history of why there's both arc.land.onto.default and arc.land.onto, but it looks like the current situation of having both is a bug that should just be addressed.