And the note from macnosy says "The only lost feature may be the ability to debug INITs," which is what I'm trying to do.
I'm late replying to this, but I recently asked the author about the state of INITs for debugging with his tool. The answer is that you still can 100%, fully debug INITs with it, and that ability was never lost.
What he was saying on that thread was that, had he gotten these tools to also run under Classic in OS X, the ability to debug INITs wouldn't work on THAT. Regardless, that September 2021 work for Classic did not complete, so there's no OS X Classic version of MacNosy, The Debugger and CoverTest (collectively known popularly as "Jasik's Debugger"). You gotta native boot into Mac OS 9.2.2 (or earlier), which is what we are all about here over at Mac OS 9 Lives.
(It is also still being sold for 100 USD. I don't think the author will change this condition.)
I was also told that, although the tool wasn't touched for 25 or so years, it was still tweaked here and there since then for a few improvements or tiny fixes.