I was looking to determine how many people with a specific tag (Facebook Ad Recruited) had unsubscribed after receiving blast emails. After calling your support team, it became clear that this isn't possible. Filtering includes anyone who did not opt in from the beginning so it is inaccurate, and it would be incredibly tedious to go through all past email blasts and individually tag everyone who has unsubscribed.
My feature request is creating a filter option for anyone who has specifically unsubscribed after receiving a blast email. This would be different from people who email opted out from the beginning and it would help me and my team determine how valuable our email list is.
Definitely a great suggestion - right now you are correct that the only way to get this is to go to your blasts and add everyone who unsubscribed to a list and update them there.