Multiple endpoints to expose different views of the same resource

Basically multiple endpoints for the same resources should be avoided. However in this particular case it does make sense.

What you can do is to introduce optional query param userId to the following endpoint:

www.domain.api.org/posts/?userId=<userId>

If this substitutes the first endpoint you mentioned that's the way to go.

I would like to add ontop of @Opal's answer.

Are these okay or am I doing it the wrong / less sensible way?

Ideally, like Opal mentioned, you would use queryParams in your url. For many applications I have build, I don't know the uids returned from the api beforehand, so selecting an item and passing it inside my url as a query parameter makes sense. But it also has the added benefit of having your key inside your url, allowing you to bookmark it, pass the url to another user and they will automatically see the same data you want them to see.

To iterate: Is your current implementation wrong? No, but ideally you would use a combination of both route parameters are query parameters to achieve this