That's weak. Be better, Slack.

For reference, I got an email back saying I was opted out and a bunch of justifications about why it's okay what they did and zero mention of the legality of opting people in by default.

I don't think this is shenanigans, just inconsistent use of English rules. I'd be floored if a judge ruled the period was pertinent here; there's no linguistic value or meaning to it (i.e. the meaning of the message is the same with or without it).

I'd be very surprised to hear that Slack was allowed to propose a free-form communication method, impose rigid rules on it, and then deny anyone who fails to meet those rigid rules. If Slack was worried about having to read all of these emails, they should have made a PDF form or just put it on a toggle in the Workspace settings. This is a self-inflicted problem.