Is it necessary to make up the missed Sunnah of Fajr after repentance, knowing that its obligation was unknown at the time?

The two rak'ahs of Fajr are a confirmed Sunnah according to the majority of scholars, and the Prophet, peace and blessings be upon him, used to observe them whether traveling or at home. The proof that they are not obligatory is the hadith of Talhah ibn Ubaydullah, which clarified that the obligatory prayers are only five.

There is no obligation to make up for missed Fajr Sunnah prayers, even according to those who deem them obligatory, because they restricted their make-up to instances where they were missed along with the obligatory Fajr prayer. One should be keen to perform them in the future. If they are missed, it is permissible to make them up, but it is not obligatory.