It has behaved this way for as long as I can recall, I know several sites built on this behavior. Ticking required adds the red asterisk to the field and registration can’t be completed unless it’s provided.
Previously I took that unchecked box to mean ‘optional at signup’. If the behavior of that box is going to change, as I know several places where providing additional data is optional and relies on those fields.
Yes, this, so different communities could ask for social media handles, etc. Asked, but not enforced.