It's much faster than using my external JSONP ever was, so don't worry that I took that app down.
Go to your followers page (https://www.tumblr.com/followers) at least once, and the list of your followers will be stored into online memory that interacts with a theme page. Then visit the page in the future if your follower count is ever smaller, and it will tell you how many people have unfollowed you, and show links to their blogs. It will even show deleted blogs, and you can check the link to see if the blog is still there or not. Click "reset Notices" to clear the list of un-followers.
It's encoded, but not encrypted, so somebody who knows the encoding might be able to see your followers. Version 2 will no longer work because I took down the external JSON.
Version 2.1 - Tumblr changed the sidebar so the unfollowers section had nowhere to be added. It's now corrected. Everything was still functional, I just had to change one classname declaration in 3 spots. It also appears at the top of the sidebar now because that seemed the easiest place where it would still run.
Update v 2.0.1 - Tumblr changed some style that was making the links too far left. I adjusted one line of CSS in this version to compensate.
I just noticed a thing. Some unfollowers may just be people renaming their main blog. I may update in future to detect renames, once I figure out a resourceful way to, but for now if 1 new unfollower coincides with 1 new follower, and also their old blog url link is 404, it was probably a name change. With their links being 404 maybe you can claim their old URLs ;)
update: v2 (improved JSON to handle hundreds of followers) version 1 will no longer work, because I changed the callback on my server. This update should allow testing for an unfollower among 500+ followers. I can't test it because I only have 108 followers, but I think the changes I've made will work.
Since version 2.1, the unfollowers box appears at the top of the sidebar. It seemed the easiest place to add it for the version fix, also it fits in nicely in the space left from where the blog list used to be. I might update the screenshots some time.
Please use wisely. Don't send threatening or harmful messages to your un-followers. This script is for curiosity purposes only.