To opt-in to something means to consent to it, to enable it, to take action to have it.

So "opt-in by default" doesn't make any sense.

However, I do see this opposite usage increasingly often, which means "opt-in" shouldn't be used any more.

Using "on by default", "enabled by default", "off by default", "disabled by default" would be clear and make sense.

I think it describes a somewhat common scenario, where it's on by default but there's an effort made to make it seem like you went out of your way to agree to it.

Like a pre-checked box on a form about your desire to receive marketing mails.