Is a #14 pigtail to a receptacle on a 20A circuit actually an issue?
The problem isn't the excess I2R drop (voltage drop) in the wire, it's that the wire is loaded beyond its ampacity, which causes it to overheat and damage its insulation as well as the insulation of other nearby wires as most insulating materials used for household wiring cannot withstand being overheated (dry steam is too hot for them, if you want to get an idea) without deteriorating significantly.

There are rules in the Code for tap conductors that can be smaller than the circuit they are tapped off of, but they apply to limited situations (such as lights, hard-wired cooking appliances, and some other types of fixed loads), never to receptacles -- the risk of an oversized-for-the-tap load being attached to the receptacle is too high.