Esoteric knowledge is something accessible to a limited number of people. As an academic researcher, many details of my work are esoteric in this sense, as there are not that many people in the world working on anything closely related. Similarly, the internal workings of an established organization often involve esoteric knowledge. Traditions emerge over time, many things are never fully documented, and people outside the organization generally have no idea what's going on.
In contrast, stack canaries are potentially relevant to a large number of developers. They are also accessible: many developers should be able to figure out the details on their own once they have heard of the idea.
Idk to be honest, it seems like you are defining esoteric in a very narrow sense, which borders on "no true scotsman", such that anything AI would be capable of producing would no longer fall within the definition.
Imo stack canaries is esoteric, at least relative to myself. On reading the first google result, I can understand the jist of the intent, but it would certainly take me time to understand the details of how it works, and to come up with an implementation. And if I didn't see it mentioned in this thread I never would have even known to search for it, and probably would have had to dig through a lot of domain knowledge to even know it's something that exists. That's the same way something can be esoteric, even if it's written about and available to everyone with access to a university library, because anyone without domain knowledge would not even know it exists or why it's relevant.
And I'm sure you can agree that if Google did not exist, probably stack canaries would be unknown and inscrutable to a vast, vast majority of developers. So if we go with your definition, the invention of Google certainly greatly reduced the amount of esoteric knowledge in existence, by making information easier to access in general. So I would argue that generative AI further reduces the scope of which information could be considered generally inaccessible,
Esoteric knowledge in the sense I described is very common in everyday life. In my work as a researcher, some of the topics are esoteric. Many of the codebases I use in my work are esoteric, because the ideas behind them only exist inside a few people's heads. The administrative processes at the university are esoteric. Many details about the gaming convention I've organized as a hobby for 20+ years are esoteric.
Other topics may be difficult and advanced, but they are not esoteric, because they are more universal. You don't need a specific context to understand them, only the basic expertise.
The internet turned much esoteric knowledge into common trivia by making information more accessible. I don't think AI will have a similar effect. At least not the public models that are mostly trained using information that is already accessible. Large organizations may achieve something with internal models, but in many cases, esoteric knowledge is esoteric because it's implicit.
In contrast, stack canaries are potentially relevant to a large number of developers. They are also accessible: many developers should be able to figure out the details on their own once they have heard of the idea.