In a previous save, i tryed to kill some companion in the Merryweather when they're knocked down. Just for check if it provides XP. It doesn't. However, there's stuff like resurrection scroll or molotov or so.
The fun things is: killing companions at this point doesn't matters. They respawn in fort joy in theyr ussual places.
Also, if you kill companions here, when you comes later back to rescue them, theres less voidwoken, so you'll get less XP from them.
Sumarizing: Kill companions for loot or let them alive for voidwoken and XP.
Idk if is intended, but as any of the options doesn't much, i post here.
Important: not checked with last update... but as i don't read anything of this in the notes, i suppose it still working