Is the use of 14 AWG on 20a circuit acceptable? If not, how long has it been prohibited by the NEC?
The answer is no this was not code in 1999. 14 Awg has been limited to 15 Amp breakers at least since the 70's. If there is a fire some insurance companies will not pay if the service has been modified without permit. Since the wire size is wrong for the breakers this could be a way out for them if a problem did occur. Breakers can be found on line for a few dollars each make sure to match the type with the panel and the brand for the safest fix.