This is not a SFW instance. NSFW communities will be tagged, and efforts will be made to keep SFW communities clean, but proceed at your own comfort level.
So the user base is not actually growing. The people who were reading old.reddit without an account had to sign up now. But that was a one-time bump in registered accounts. It does not necessarily mean that the user base is increasing.
They recently started requiring logins, especially for old.reddit
And that is the only reason the userbase is rising, and the user base rising is the only reason they started requiring accounts to view the site.
Numbers must go up.
So the user base is not actually growing. The people who were reading old.reddit without an account had to sign up now. But that was a one-time bump in registered accounts. It does not necessarily mean that the user base is increasing.
Don’t they always?
No, it used to be possible to browse the site without an account.