Sorry for the current state of sisu in Debian (and to some extent in Ruby more generally)...
I have a version of sisu working with ruby 3.0 & 3.1 (it can be run straight from the command line) ... but not made a deb of and built with Debian dependencies. Certainly the latex/pdf output has not been tested.
Also there remain a couple of issues to check.
To clarify, the version that runs "straight from the command line" is run without special installation straight out of the project ./bin directory against the document(s) to be generated ...
This currently works for the html and epub at least (where ruby is the only dependency) ... latex to pdf should work once the positions of the latex.stys that are depended on are sorted out correctly. (not even the ruby gem is update at this moment).
See below
I am not against Debian-live shipping documentation independently but will try to make sure sisu works as it should, (though I may request help packaging it once at that point).
I apologize, l have somehow been ignorant of the fact that sisu was still in use by the debian-live project. I think knowing this would have made me spend a bit more time maintaining it. There has been nothing to stop me from knowing, so it is my fault. Sorry.
You may follow up on my progress here or directly with me.
A note on the D (dlang) version:
I have a D (dlang) version that I have been actively developing under the name spine or sisu-spine.
(which is considerably more up to date in some of the output it produces than sisu but I have not sorted out how I wish to release it and am content to develop it.)