This behaviour is not only spammy, but disrespectful in this day and age. Someone posts once, and seems to get emails about the whole activity, comletely spamming the mailbox with unrelevant discussions. It is made even worse, because if you - by mistake - happen to authenticate with a work email (for example, by logging in from a wrong browser and authenticating through github, then company policy and/or mail filters may prevent you from following the links to reply or unsubscribe and require a manual visit to the site in order to find the subscription permissions. Which, as you are surely aware, no one does any more - if I receive an unwanted email, I automatically block the sender.
Thanks for starting the discussion, though I don’t think the situation is that dire. There is not that much new topics and they will be bunched into one thread by most inboxes. If you want to unsubscribe to new topics it’s easy to do via <your_user>/preferences/emails endpoint.
Let’s see if more people have an issue with the default and we can rethink it.