What do you mean by "how"? The code is freely available, you can spin it up at will too in just a few minutes. When they disappear, sometimes it's because the maintainer doesn't want the headache, and sometimes it's because a they're threatened to remove it.

I assume the source code repo is still up somewhere, so I assume it's the domains that are being taken offline, not the code, and anyone can then trivially spin up their own instance.