Here is the codebase. If nothing else, if you wanted a foundation for a scheme-based activitypub system, you could start looking at this (one that could be packaged and usable by the Guix enthusiast crowd, even):
https://gitlab.com/dustyweb/pubstrate/-/tree/master/
And the test suite:
https://gitlab.com/dustyweb/pubstrate/-/blob/master/pubstrate/aptestsuite.scm
The pubstrate repo is really three... no wait, four things:
- An ActivityStreams library
- An ActivityPub library
- An ActivityPub demo application
- An ActivityPub test suite
All of these were cobbled together by me under tremendous strain and without enough energy or support during the last year or so of the activitypub standardization process. There are, nonetheless, some good ideas in there, I think. It's probably not compatible with most current activitypub applications (and yes, that's an interesting piece of commentary itself), but it could be made so without too much work I think.