I think it's because many servers, apparently including the one AO3 uses, have started blocking (or use a third party platform that has started blocking) requests from our servers for insecure (http) links. The code change is to start using https again, but the personal machine you read email on can load the http version just fine.

The reason it's not working for the readers of this post yet is because the change is still waiting to be deployed outside of canary. If you have canary turned on, it works as expected. (And if you turned on canary in the past and it's not working, the relevant cookie expired and you need to turn it off and on again. That's the demographic I found myself in earlier this month.)