There is a lot of information on that site. Do I know people that have used extension cords for level 1 charging? Yes.https://www.esfi.org/resource/extension-cord-safety-tips-478 said:
Is the biggest concern replacing permanent wiring with an extension cord, I believe so.
Maybe this link will be instructive: Have you ever used an extension cord to plug in your car? Twitter poll results
In closing, don't do it you can burn your house down, but most people do it at one time or another.