There’s now a page where you can see who you’ve unfollowed from your reports.
You can look by year, month and day.
If you’ve got a super huge account, it might be a little slow, and some of the really old data might be missing (for example, we didn’t start recording the reasons for unfollowing until a few months back). Since you’re digging through archives, it’s a part of the system that’s probably not worth optimising the heck out of.
Those caveats aside, it’s a reasonably good summary and should make it easier to refollow people, if you so desire.