Ok, I think I understand your point now, there is a comment talking about profile information that probably make you concerned but that's not the intention of the original comment.
What is being asked is to know who is playing your game if this person is logged in, and that's all. This information is public (if the person decides to publish something).
Personally, I think it's a reasonable request and can cause more good than harm.
About the implementation, instead of a opt-in, itch.io could simply warn users that the username is public. And because is possible to change your username, if someone wants to change before this new feature is released, they could simply change their username. What is the point of the username anyways? Display it publicly right?