It's up to the store owner to actually default to "agrees to marketing". I'm not sure if Shopify is to blame when it's the owner that used an illegitimate opt-out for that setting instead of an opt-in.
And of course, follow-up mails for abandoned carts are an optional setting too.
The default for “Agrees to marketing” controls if the box defaults to checked on checkout, so I do think if the store disabled that you wouldn’t be subscribed.
My theory is it started by accident- if you get a notification that says, “this checkout, this email, agrees to marketing: true”, it sure reads like an opt-in, and it used to be reliable. But it’s not anymore, because your email is already attached to the checkout when its created.
“Agrees to Marketing” pre-dates the global Shop session by years, it’s plausibly an ecosystem bug; one with no real motivation to solve until customers start talking (more)
And of course, follow-up mails for abandoned carts are an optional setting too.