It looks like you asked a few months ago [1] and nobody seemed to know. The last archive.is snapshot is from 2021 [2]. I had never heard of the site. Someone is still paying for the domain and have the DNS at Amazon. Their MX points to "localhost.admin.com" and that does not have any A records. The sub-domain book.admin.com does not have any A records either. Their SPF record states they do not send email. The domain registration expires 2031-11-16T05:00:00Z. Just guessing but maybe they lost whomever was supporting it and did not want to lose their Google ranking?
It appears that domain was primarily used to promote a book.
[1] - https://news.ycombinator.com/item?id=41254153
[2] - https://archive.is/*.admin.com