Such a system doesn’t prescribe which action from that set, but in order for it to contain supererogatory actions, it has to say that some are more “morally virtuous” to others, even in that narrowed set. These are not prescriptive moral claims, though. Even though you follow this moral system, a statement “X is more morally virtuous but not prescribed” coming from this moral system is not relevant to you. The system might as well say “X is more fribble”. You won’t care either way, unless the moral system also prescribes X, in which case X isn’t supererogatory.
Many systems of morality are built more like “do no harm” than “do the best possible good at all times”.
That is, you are morally obliged to choose actions from a particular set in some circumstances, but they do not prescribe which action from that set.
Such a system doesn’t prescribe which action from that set, but in order for it to contain supererogatory actions, it has to say that some are more “morally virtuous” to others, even in that narrowed set. These are not prescriptive moral claims, though. Even though you follow this moral system, a statement “X is more morally virtuous but not prescribed” coming from this moral system is not relevant to you. The system might as well say “X is more fribble”. You won’t care either way, unless the moral system also prescribes X, in which case X isn’t supererogatory.