I am the developer of the “other” ActivityPub library for go: https://github.com/go-ap
Me and Cory (the maintainer of Go-Fed) have been in touch for a long time when I got involved with the ActivityPub work, but I think his interest has wandered into a different direction and I don’t think he’s actively involved with the library any more.
Unless other developers pick up the slack and take over the maintenance I think the only option for other people is to look at alternatives. If you want a compatible API, GoTo Social is maintaining their own fork of go-fed (afaik). If you’re looking for something closer modeled after the spec documentation itself, you can look at my humble contribution mentioned above. I personally would welcome any support from developers or users (especially in the form of documentation that new users would benefit from).
For anyone wondering about using go-fed vs go-ap, you might also want to read Gitea’s rationale for choosing go-ap for federation: https://gitea.com/gitea/go-fed-activity/issues/1
One upside of go-fed is that it already supports the ForgeFed ActivityPub extension which is needed for Gitea federation, but it wasn’t too difficult to write our own ForgeFed implementation using go-ap with Marius’s help.
Seems dead: https://github.com/go-fed/activity/commits/master
The demo app doesn’t actually build you a working app, I just went through the tutorial the past two hours.