> Now Reddit (one of the largest sites on the internet) does what Mastodon does and forces new users to choose before using an account.

It doesn't though - creating an account doesn't tie you permanently to a particular subreddit. I would guess, without checking, that they will suggest some subreddits to subscribe to, and if you try to sign up from a particular subreddit's page then maybe it'll subscribe you to that subreddit. But if you joined the wrong subreddit at the start then you're fine - you can unsubscribe from that, and subscribe to another, and your account is still the same account.