Most of the forums I patronized left their index open to the wider public, maybe also let the wider public read posts, and almost always blocked user profiles (read: treasure troves of personal information) without logging in.

User profiles were indeed blocked to the public on most forums, but I very rarely found that a problem

Change that maybe to definitely, I guarantee you that very few were auth-walled

> Extremely few forums required an account to read them 20 years ago

The few forums I currently frequent don't require an account to read most of the threads.