It had half a terabyte of RAM, sand they’re not saying what it cost. It could also be that the demand for that model was so low, they wouldn’t list it anymore. If you needed that configuration, you could probably special order it.
No consumer needs more than 16/32GB of RAM on a Mac. Some gamers are pushing 64GB RAM, but I haven’t heard of anything but fringe benefits for doing so. The people who need 512GB of RAM are doing some high end video production — think like Pixar type applications. Making these extremely detailed models and rendering them in 4K for a movie. That kind of thing.