At the bottom of “Activity Summary” emails you get this notice:
If you click on that link you end up here:
Wouldn’t it be better to change the copy to something like (addition in bold):
“This summary is sent from SITE NAME when we haven’t seen you in a while. To unsubscribe or change your preferences click here.”
And then provide a UI like this:
This would give people the option to change their activity summary frequency, in addition to unsubscribing from this feature and/or all email from that Discourse site.
(The copy & UI might require some word smithing. I just threw this together to communicate the basic idea.)