Let's ship 5.x with a deprecation warning for this and all the methods that use it. We won't deprecate or remove it in 5.0, we can do it in the next minor version that follows.

Once StreamFactory is removed, there will be less need for ByteBuf directly and we could replace it with methods that use Java's ByteBuffer class. It's currently still used in RawBsonDocument and RawBsonArray.