I believe any website that is behind the walled garden should be deindexed by Google and other search engines. Whether it's a newspaper, Q/A websites, social media, or anything else. It's only fair. When users search for something, they should only get results of web pages whose content is accessible to all without having to sign up for an account.
Ideally Google would have a search filter that lets you block paywalled sites from your search, if that's what you want.
The vast majority of the time, someone searching something on the internet isn't willing to pay to see it.