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

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."

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