United used to offer discount Twitter fares available nowhere else (called ‘TWARES’). Those ended because they ran afoul of United’s contractual obligations to offer their best fares to everyone.
15 years ago most US airlines offered email specials. They’d be valid for the coming weekend, dumping unsold inventory, and ensuring that the allowable travel dates didn’t coincide with business travel times. United had them. So did American. So did US Airways. Continental used to refer to them as CO.O.L. Travel specials (COOL stood for Continental On Line). We don’t see those anymore.