I was contemplating a E_WARNING actually.

Note that there is nothing "suddenly showing up" here. This ($out) has never been supported before so there is no BC concern here.
And when you are developing and somehow using SECONDARY_ONLY read preferences.. I think you really do need to get-in-your-face that YOU ARE DOING IT WRONG. Especially considering the $out collection is not replicated afaik so your next read on it is gonna fail.

Hiding it in a MongoLog which spews out bucketloads of totally random crazy stuff has absolutely and utterly no meaning to any developer at all, and never will.