Complex sentence: Secular portraiture was a rarity in European art before 1430, when the format did not exist as a separate genre, and was only found infrequently at the highest end of the market in betrothal portraits or royal family commissions.
Split into simpler sentences: Secular portraiture was a rarity in European art before 1430. The format did not exist as a separate genre and was only found infrequently at the highest end of the market in betrothal portraits or royal family commissions.