I think you might be mixing them up?
Opt-in means that the user needs to actively decide to use the feature, it's OFF by default. That's currently the case for atuin (happy user without sync here).
Opt-out would mean the feature would be ON by default and users would need to actively decide NOT to use it.
You actually have to go and register an account or log in for sync to happen at all
Otherwise it’s offline, by default, right after install
if you have someone outright cloning your entire project just to skip this opted-in privacy gap feature, it's a good indicator it's an issue to be mindful of!