Decayed does not create any attacking requirements. You may choose not to attack with a creature that has decayed.

Once a creature with decayed attacks, it will be sacrificed at end of combat, even if it no longer has decayed at that time.

Decayed does not grant haste. Creatures with decayed that enter the battlefield during your turn may not attack until your next turn.

Decayed represents a static ability and a triggered ability. "Decayed" means "This creature can't block" and "When this creature attacks, sacrifice it at end of combat."