I am committed to supporting ScalaSTM for the indefinite future. I haven't done much development lately because I implemented all the features that users requested, and I know of no bugs. I haven't needed to repackage and deploy because Scala 2.10 is binary compatible across minor versions. (I did push 2.11.0-M3 builds recently, but I didn't update the website to reflect them due to their M status.)
Since the STM doesn't need special access to the compiler or libraries, the only compelling reason to include it in the standard library is if other standard library components rely on it. That's not currently the case, so it is better to keep it outside. It is hosted on Maven central, so downloading it is as easy as including it in the build dependency list.
It is possible that macros could make the syntax more concise, but I have no concrete plans to experiment with that.
Thanks for your interest,