"Graveyards are full of essential people" (Bill Clinton). If there is someone who has information such that a handover is needed you as management screwed up by not preventing it in the first place. The person might have died and then there is never a handover possible.

Paying severance is very common even in the US where it isn't required by law (though generally not as long as Europe requires). Severance is what you pay people AFTER their last day.