Thanks everyone for your inputs.
It seems like there are stronger preference towards the slash syntax, but there are some concerns around backward compatibility of existing builds. I think we can address backward compatibility by keeping around the old syntax for the duration of sbt 1.x series.
There’s also been concerns around introducing this change late in the game, so after discussing with Dale, we decided to introduce this as a form of a plugin. This way you can start playing around with it without waiting for sbt 1, and get better feedback. https://github.com/sbt/sbt-slash