Thinking about it, I suspect that part of the reason they didn't opt to include Vengeance paladins is because they're still working on the oathbreaker flags. There may be flags for Devotion and Ancients oaths seperately, but they're unrefined at the moment, so Larian included them because they're, in their mind, similar enough that if there's a mix up in the oathbreaker flags, it wouldn't lead to something like paladins becoming oathbreakers for showing mercy.
Vengeance has a bigger problem with them being about bringing justice without mercy to the "greater evils" in a game without alignments or a clear big bad from the start.
And if they plan to reenable the gods selection, they need an actual "for evil god" subclass that isn't Oathbreaker and while Vengeance can be twisted to fit via "my god sworn enemies", it might get a bit complicated to handle code-wise.
