"Came out" refers to when they became in widespread use. That was in the 70's.
As far as low power I can find nothing that indicates that it was ever ok to put tin foil in however I do know that in some cases you might put metal to specifically block cooking. So perhaps with a low power microwave there was no sparking etc. (I can't find anything on that and I don't see the link you sent showing that (which page is it on in the cookbook?).