Shirayuki no Koi: Taishou x Omegaverse

The time is Taisho. Yukito, a beautiful omega born as the illegitimate son of the head of the family, is allergic to suppressants, so when the heat comes on, he is consoled by Keigo, the butler. The two have been by each other's side since childhood and although they're secretly in love with each other, Keigo has his reasons for not being able to become Yukito's pair... (Source: Futabasha, translated)