Turns out they are prohibited for new gTLDs. https://www.icann.org/en/announcements/details/new-gtld-dotl...
https://news.ycombinator.com/item?id=31689505
https://news.ycombinator.com/item?id=32804189
Also: http://ai./ works (at least in Safari)
http://ai doesn't open in a browser for me, while http://offshore.ai/ does work.
Turns out they are prohibited for new gTLDs. https://www.icann.org/en/announcements/details/new-gtld-dotl...