Delta exclusively tested offering SweetWater 420 on flights between ATL and LGA. They have supposedly expanded offering 420 on the following routes to/from Atlanta:

• DCA – Reagan National Airport (DC)
• FLL – Fort Lauderdale-Hollywood International Airport
• IAD – Washington Dulles International Airport (DC)
• MCO – Orlando International Airport
• MIA – Miami International Airport
• MSY – Louis Armstrong New Orleans International Airport
• TPA – Tampa International Airport
• LGA – LaGuardia Airport (NY)

Though evidently it hasn't been available consistently on all routes based on the OPs experience.