I think there's two points to make here. Firstly, despite the title of this forum thread, supporting Pebble devices in your app is absolutely fine. This rejection was for mentioning Pebble in the metadata. Secondly, at this point it could be a case of a single decision by a reviewer rather than new instructions coming down from above (although there are comments in the thread suggesting other rejections are happening for this).
However, this rule has been in the guidelines for a long time and is completely unambiguous. Still, it feels like a bit of a disappointing move on day one of the watch.
Do you use the Dropbox API in your app? If you do then you need to read this. As Dropbox themselves say, uptake on their new API has been slow and while shutting down the old API is certainly one way to push developers into action, it's going to mean a chaotic October for those who inevitably ignore this! There's still plenty of time to get things updated though, don't leave it until it's too late.