In Scala I see such feature as object-private variable. From my not very rich Java background I learnt to close everything (make it private) and open (provide accessors) if
I don't think it matters too much, since any changes will only touch one class either way. So the most important reason to prefer private over protected over public doesn't apply.
Use private[this] where performance really matters (since you'll get direct field access instead of methods this way). Otherwise, just settle on one style so people don't need to figure out why this property is private and that one is private[this].