the prefix will also be used in other circumstances where, for example, the holder of a valid licence is returning to flying after a significant absence and is undergoing renewal training.
The PPL / integrated course is the only training that involves supervised solo flying. Everything else is 100% dual training. Therefore, the prefix "student" should only be used for solo students on a PPL / integrated course. In all other cases the student will either be accompanied by an instructor or hold a licence.
Licence holders may be rusty however, the prefix student is not designed to warn people that the pilot may not be 100% capable of completing the flight safely.
It is designed to let ATC know that it is a student who lacks experience (not skill) and should not be given "unusual clearances" eg orbit on final which they may not have the experience to refuse.
If people other than solo students on the PPL / integrated course use the prefix then the benefits will be diluted.