By default, it now does unpopulate_light. I'm now reasonably confident the avoidance mechanisms for not deleting important stuff are good enough, so it will wipe chroots of anything that doesn't belong there.
See the man page which was updated accordingly. Some fringe case scenarios may need to use preserve if they have chroot data that's not accounted for otherwise. In general, it should have little impact on current users of proot I know of, most of which actually use unpopulate* already.
