What is the ruling of the one who does not pray the two Rak‘ahs of Fajr, knowing that they are a confirmed Sunnah and not obligatory?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

Amongst the confirmed supererogatory prayers, as you have mentioned, are the two Rak‘ahs of Fajr. The Prophet, sallallaahu ‘alayhi wa sallam, used to observe them. It was narrated that ‘Aa’ishah said: “The Prophet, sallallaahu ‘alayhi wa sallam, was never more regular and persevering in offering any Nawaafil (i.e. supererogatory prayers) than the two Rak‘ahs of the Fajr prayer.” [Al-Bukhari and Muslim]

It was also narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: “Pray them (i.e. the two Rak‘ahs of the Fajr), even if you are being attacked by a cavalry.” [Abu Daawood]

The Prophet, sallallaahu ‘alayhi wa sallam, used to repeatedly encourage praying them saying: “The two Rak‘ahs of Fajr are better than this world and everything in it.” [Muslim]

Scholars of jurisprudence stated that it is recommended to make them light due to the Hadeeth reported by ‘Aa’ishah in which she said: “The Prophet, sallallaahu ‘alayhi wa sallam, used to make the two Rak‘ahs before the Fajr prayer so light that I would wonder whether he recited Umm Al-Qur’aan (Al-Faatihah) or not.”

Accordingly, a Muslim should observe these two Rak‘ahs due to the Hadeeths mentioned above. Whoever does not observe them will not be sinful. However, he will have done something disliked and will have missed a lot of good things if he abandoned them intentionally.