Besides the fact that the extension is broken already I don’t think so: AFAIR Message Stream also depends on some templates in xwiki-platform-web (e.g. for notifications). That I was thinking of just removing from XS after moving the module itself.
Then I’m wondering if it is really worth creating an attic repository, one can always go get the source in xwiki-platform git history or in the Maven repository.
Re attic, I think it’s good to do it as it makes it simpler if someone wants to resume work on it (rather than track the sha1, etc). But I’m also fine if we don’t do this.