étape de fin / étape de nettoyage

The reason it's templated this way is a number of reasons. First, it's so that a player doesn't get the chance to cast it as an instant during their opponent's end step and then untap and attack with the creature, thus mostly avoiding the downside of casting it as an instant and making it into a more flexible Shallow Grave.

It also means that Necromancy's trigger (and all other triggers that occur during the cleanup step) occurs after damage is removed from permanents, "until end of turn" and "this turn" effects end, and players have discarded down to 7 cards.

In a weird corner case, the functionality of the triggered ability is also preserved around effects that say "end the turn", since prematurely ended turns still have a cleanup step.