> Supporting a flip device with vertical folding would be a new form factor for GrapheneOS. The main use case for those is a tiny phone to use with one hand in the folded form factor.

So, I've been using flip phones now exclusively since the Samsung Z-Flip3 (and will never go back to slab phones unless the flip segment disappears), but I almost never use the outer screen. Maybe that is just me, but I have a feeling it's pretty common.