Is Strong Back legal in Standard, Modern, and Commander?

Strong Back is legal in: Legacy, Vintage, Commander.

What type of card is Strong Back?

Strong Back is an Enchantment — Aura. Its oracle text reads: "Enchant creature Equip abilities you activate that target enchanted creature cost {3} less to activate. Aura spells you cast that target enchanted creature cost {3} less to cast. Enchanted creature gets +2/+2 for each Aura and Equipment attached to it."