HATEOAS is more about a standard, discoverable and consistent way to navigate around resources in a RESTful fashion. Which is nice, even if it didn't really catch on to a large degree. I think that's somewhat orthogonal to he point you're making.

I think your point is good though, that an ephemeral toast message is a cheap way to avoid having to manage and expose the underlying state. Exposing that via a HATEOAS REST API would be even better!