Did commercial airliners have microwave ovens in the past?
Generally speaking, airliners do not have microwave ovens for safety reasons. I could not find any reference where commercial airplanes used to have microwaves. Probably some smaller business jets have them.

Some flight attendants mention that they can warm your food, but some say that they will not for legal and health reason.

As also mentioned by sweber in comments to the question, meals are not heated at the airport. Airline catering companies like LSG Sky Chefs prepare the meal near the airport and deliver it to the airplane. Inside the airplane, they use convection ovens to heat it.