I'm not so convinced that xtrmsg.sbx was ever included in any release package, outside of the SOSLIB. And it doesn't seem to be documented in the XCALL Ref either.
I may have forgotten now, but I think that it, like most of the things in the SOSLIB, was intended to be adopted by developers, either as-is, or with your own modifications, and packaged with your application files.
Even if we were to include it, there would be the danger that we might tinker with it, so I'm thinking it is best that you continue packaging it with your application.