Similarly it’s less common, but still not uncommon, to have provisions for leavers. Either some kind of prenegotiated basis to determine how you calculate fair value and buy back the share. Or I have seen something that was more like a reverse vesting in that the ownership expires X years after you leave.
The benefit of sorting all this stuff upfront isn’t just to avoid negotiating at the worst time when emotions are high. The game theory aspect of it requires you to balance not getting screwed with what seems fair if you’re the one that leaves.