Lost of discussions on this in the community. The answer is not simple. It first of all depends on geographies and on the business your are in and your targets.
It's very likely Marketing newsletter are not operational, but product newsletters can be. Event registration confirmations are operational, but event invites are not.
Hey John, I'd look at a number of things -- including how long ago someone unsubscribed, and the overall number of people you're looking at "losing". If you have a business relationship with these people there is some gray area as to how you can message them, but you should still be getting them to opt-in, and not just re-adding them to your lists and expecting them to update their preferences if they don't want to hear from you.
When you're getting them to opt-in, if they aren't familiar with your company name or used to receiving/opening/actioning emails from you, it's highly unlikely that you'll see a huge return from a mailing. That's why I'd look at how long ago they unsubscribed, and if the overall number of people is worthwhile to do a mailing like this.