A central point of REST is that any resource (e.g., user) can support an unlimited number of different representations. This isn't just true on the level of, say, XML vs. JSON but also formats which embed and do not embed subordinate/related entities.
> You could do that with query params sure but IMO that's a workaround with what the frontend really needs, a declarative way to let the API know that for this specific request, this is the exact data I need and its shape.
How are query strings a workaround and not a declarative mechanism?
(I mean, it would be better if HTTP had a verb that was like GET with a body so that you could define one or more media types that could be used to specify details of the resource representation sought, and that's what the QUERY draft [0] is about. And I can see the view that query parameters are a workaround for the lack of a QUERY method. But that's a slightly different story.)
[0] https://www.ietf.org/id/draft-ietf-httpbis-safe-method-w-bod...