From what I understand, though, this is a deprecated API. No sunset announced yet but as we're striving to develop for the future, is there any other endpoint we could use? Or will this endpoint be sunsetted eventually?
That endpoint isn't planned for sunset in the near future; many of our APIs currently have new versions in development, which can be found in our new docs here, but that doesn't mean that all of the exisitng APIs are deprecated just yet.
There currently isn't a new Contact Lists API, so for now the endpoint you've linked to is the best way to get the contacs in a list. Stay tuned for updates by subscribing to our changelog.