Arcbound Slasher

• WOTC Official Reminder Textofficial rules
  Modular — This enters the battlefield with N +1/+1 counters on it. When it dies, you may put its +1/+1 counters on target artifact creature.

• WWizards of the Coast2021-06-18
  Official Ruling
  If this creature gets enough -1/-1 counters put on it to cause it to go to the graveyard, modular will put a number of +1/+1 counters on the target artifact creature equal to the number of +1/+1 counters on this creature before it left the battlefield.
• WWizards of the Coast2021-06-18
  Official Ruling
  If a creature entering the battlefield has riot but can't have a +1/+1 counter put onto it, it gains haste. In the case of Arcbound Slasher, however, it's toughness will probably be 0, and it will be put into its owner's graveyard before it can attack.
• WWizards of the Coast2021-06-18
  Official Ruling
  Riot is a replacement effect. Players can't respond to your choice of a +1/+1 counter or haste, and they can't take actions while the creature is on the battlefield without one or the other.
• WWizards of the Coast2021-06-18
  Official Ruling
  If you choose for the creature to gain haste, it gains haste indefinitely. It won't lose it as the turn ends or as another player gains control of it.

Rulings via Scryfall