In an earlier thread I was claiming that http://ai/ was no longer operating, but I belatedly learned that this is because systemd's DNS resolver has hard-coded a refusal to look up addresses with a single label from servers outside the LAN. I misinterpreted the lack of results as implying that it had been removed upstream.

https://news.ycombinator.com/item?id=27157712

So, (1) there is another site that is as short as the one you mentioned, and (2) Linux systems that default to using systemd-resolve won't be able to see either of them!