Square won't show the email if you're not opted in. But it is possible to download the email list, so merchants might be grabbing it and then emailing through other tools, or re-uploading to Square as new customers.

I think they made a change so opting in is across their network, rather than per merchant.

Opting out of one also opted out of all at one point (I never get these), but they might have changed that too, wouldn't surprise me. I've definitely seen an increase in customers who are opted in.