They also don't know how to make the blue glass used for chuches those formulas were lost as well. Copper is the substitute and much cheaper than gold but gold is still used for certain hues as the hues created with copper and gold do not match 100% and also have a different saturation. But in today's cheaper is better world very few people and even less mfg care about such things.
Something that might interest you is that Tiffany's favrile glass was patented in 1894 and with some luck the patent explains the process much more precisely. I also found many sources that claim that Tiffany himself invented the process and that it is based on an old roman techniques.