1. This was the existing behavior and this pull request just documents it.  Therefore, it LGTM.
2. The system property doesn't have a good reason to need commas, since those are usually defined locally and can be OS dependent.
3. The sbt.boot.properties file is used across OSs and an OS-dependent syntax is not viable.
4. The system property is substituted in the sbt.boot.properties file and therefore uses the same syntax.
5. The comma separator will always need to be supported for compatibility.
6. So, while this particular system property could be special cased, it is unlikely to happen.
7. This isn't intended to be a new standard and I don't see its use being expanded in sbt.