Unfortunately it seems like it's not maintained anymore
Jard is a tool that is simply done/complete/ready.
JavaScript as a developer culture seems to have convinced a lot of folks that if a package isn't updated at least a few times a year, it's "dead".
RubyJard looks cool, but the repo is marked as "Archived" and the last commit predates Ruby 3. And, in fact, it looks like there are issues with Ruby 3.2 and RubyJard.
Most software cannot really ever be "done", if only because no software operates in a vacuum. Nearly all software relies on a language, a standard library, an OS, direct dependencies, indirect dependencies, etc. The only way your software can be "done" is if it relies on absolutely nothing, and very little useful software does that.
Well, that sucks. Still, I urge people to clone a copy, update the pry dependency and give it a try because I've never run into any issues and it's at the center of my debug game. I'd be devastated to lose it.