So: here’s the thing.
BBEdit 11 (maybe even 10, but I think 11) took away one of the AppleScript functions my Markdown extensions depended on, and while there’s theoretically a fix available, I haven’t been maintaining that. There’s a lot of reasons why—for a while I wasn’t using BBEdit much at all, although I’m using it more again these days (it turns out to be really good at maintaining huge honking static web sites)—but the bottom line is that the Markdown Extensions package is broken and the chances are I’m not going to fix it.
Here’s the good news: just about everything my package did, as well as a similar
OS X macOS Services package I used to maintain, is done better by Brett Terpstra’s Markdown Service Tools.
Seriously. It does Markdown and MultiMarkdown to HTML and RTF, HTML to Markdown, and a lot more. And it works every place that Services do, not just in BBEdit (obviously, it works in BBEdit, too.) I really suggest you migrate.
As for my other open source projects: unsurprisingly, they’re all on GitHub. I may move the old BBEdit stuff to my GitHub, too, with “DOES NOT WORK PLS FIX THX” stamped in the README files, but no ETA on that. (I’ll update this page if I finally do that, though, I promise.)