Custom DSLs that Ai's don't know about are a bad idea, and also generally for many reasons because they are hard to get right. You are going to have to feed the context in on how to use them for every message. Wasp may be ok here, as they predate the inflection point. They still have a knowledge gap / usage issue. No one really uses or wants the DSL, so there are few humans who can be in the loop to make sure it works as intended.
Btw, AI actually works great for it. I am sure part is that the Wasp's DSL exists for some time now, but it actually worked well for the very start, because the DSL was quite simple (similar to JSON) and AI knows how to generalize very well.
So I wouldn't discourage people from writing DSLs because of AI -> AI can understand them very well -> but for the reasons of missing out on all of the benefits of using a strong host language and doing it as an embedded DSL in it. If you are doing your own, completely standalone DSL, you will need to implement a compiler, editor extensions, LSP, maybe module system if you need it, maybe package system/manager if you need it, ... . Although when I think about it, that is also easier now with AI, than it was before! Hm yeah actually maybe custom DSLs are a good idea these days, with AI doing most of the job for you. I still wouldn't go back to custom DSL for Wasp however because biggest thing for us is probably familiarity -> custom DSL just scares people off.