What I did, for the Habul XPI, was to unzip it and edit the install.rdf file, to increment the MaxVersion number for Thunderbird. In this case I just increased it to 68.* or something so it would last.
The code itself seems very elementary, though I'm not an XUL programmer. I can't imagine it would be difficult for an experienced programmer to fix it up for current TB builds. That having been said, I'd also be in favor of an external tool that only needs access to the IMAP folder itself, though admittedly it might be more time consuming -- but the reporting is what I want. If I have to go around and avoid addons because of API changes, fine.