Individuals cannot sue under CAN-SPAM Act and I don't think any state Attorney General is going to bring a suit on my behalf because they didn't accommodate a plus alias.
https://consumercomplaints.fcc.gov/hc/en-us
The technical details as to why they are failing to unsubscribe you does not matter at all. What matters is, you've unsubscribed repeatedly and they are failing to register your intent.
'+' has no special meaning. The relevant standards only say that senders can't dictate how a system encodes its addresses. No law is in place to compel that behavior. If that were the case I'd love to force ISPs supposedly selling IP services to allow unencumbered use of port 25, residential servers, and transport protocols other than TCP and UDP.
> I have several services I will never be able to unsubscribe from because of the latter—and yes, I tried manually URI encoding it
Edit: Downvote me all you want, they're still breaking the law… In the US there's the CAN-SPAM act (https://www.ftc.gov/tips-advice/business-center/guidance/can...) and nearly every other country has similar laws. Most of them require one-click unsubscribes to work.
It's still failure to comply even if it's due to negligence/ignorance/incompetence.