What does HackerNews think of join-monster?

You could use https://github.com/join-monster/join-monster, but setting that aside it's usually enough to use Dataloaders that resolve based on some source ID, with some server-local caching on top if latency becomes an issue.

REST is not a strict specification and it's not a single implementation, you can just start doing it.

That said, I wouldn't recommend going the allow everything flexible resolver way like GraphQL: it's terrible for performance (eg. most APIs use N+1 queries unless you have something like https://github.com/join-monster/join-monster), the complexity of the codebase skyrockets and having to specify all the fields you want is not exactly ergonomic in most situations.

Assuming something in the same vein as fizx with the addition that it's probably very similar to what Join Monster tries to solve: https://github.com/join-monster/join-monster