Twitter and Reddit still can be indexed, but they've also become increasingly hard to use without an account. Reddit doesn't let you fully expand threads when you're unlogged. Twitter limits the amount of things you can read and shows a modal. Both of them heavily limit usage on mobile devices without installing an app.

Sure, an account is free but might require giving information you don't want to give. Twitter asks me for a phone number a few minutes after creating an account, even if I don't post anything). Reddit at least lets you skip giving an email.

Sure, there are workarounds such as using lite versions (old Reddit, mobile Twitter), but that's not known to all people coming from a search engine.

It feels as if HN are the only one that's not a partially walled garden yet (and Wikipedia of course).