The horizontal slot on a 20A outlet and the corresponding prongs on a 20A plug are indeed there to identify the circuit as a 20A circuit and prevents 20A devices (like treadmills for example) from being plugged into 15A circuits.
There really isn't any inherent safety issue with a custom cord having a 15A plug and 20A IEC in and of itself...what I do have a problem with is if frequent nuisance breaker trips now lead the homeowner to change out the 15A breaker to a 20A breaker because of the use of this "cheater" cord. This is a big no-no because it would then overload the 14AWG wiring to the outlet.
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