Thanks for your mail, Tobias! I'm very glad that Soundcloud is finding mtail useful.
These are good questions and I don't have a great answer for you yet, but thanks for raising them.
I try very hard to not make breaking changes to mtail. Every tagged release has to pass all the tests, which while not perfect, at least confirm that the language and exports are consistent. After we pass the rcX candidates and really launch 3.0.0 then there will be a very safe assumption on nonbreaking change, I expect. But like all these things, we need volunteers to report problems when they happen.
I don't believe that you will have a problem upgrading your mtail to the latest rc tag, because I have high confidence that it does not include breaking changes, but of course I would like to know if you do have any. Is there any way you could incorporate the latest tag into your release testing so that you don't end up finding a lost instance 15 months later?
I'm not keen on publishing a changelog because that's the purpose of the revision history, IMHO. But if there's demand, I can try. Or, appeal to the generosity of volunteers to help.
The releases are proper, already, but I haven't got a good definition of what it I'm trying to reach before tagging 3.0.0. I can't promise a time bound on doing so, but I should review what's missing and commit to a minimal set of issues to fix before we declare this no longer a release candidate but a "real" release.
Again, thanks for starting the conversation. What do you think about what I've suggested above? I'm happy to hear any and all criticism.