Don't have any screenshots handy but I'm pretty sure the cutoff is 0.1 atm. Or a tenth of Earth's atmosphere.
I'm fairly sure I've encountered landables at 0.09, and non-landables at 0.11/0.12.
There's also the occasional planet with less than 0.1 atm that's non-landable due to other features that aren't present in Odyssey, like very large-scale volcanism (Io-type worlds, mainly) or thin-atmosphere water worlds.