Simple: send a letter every month with a link that says “Click here if you still want to receive this newsletter”. Then remove everyone from the list who hadn’t clicked it after a week.
“Why did you stop sending it to me?”
“You didn’t say you it.”
Yes I did! 15 times!
Well, not this month.
It’s like consent for sex is supposedly supposed to be like today for some inexplicable reason.
It doesn't have to be every month - each time someone reconfirms their interest, you could wait longer before asking again.