I wouldn't say it's common but it's not unheard of either - if you search for "牡蠣ラーメン" there are several shops in Tokyo serving oyster ramen, including some that specialize in it! It's often combined with other shellfish and chicken/pork to make a shellfish-forward broth
I've never heard of oyster ramen before! Have you ever seen it somewhere else?
I wouldn't say it's common but it's not unheard of either - if you search for "牡蠣ラーメン" there are several shops in Tokyo serving oyster ramen, including some that specialize in it! It's often combined with other shellfish and chicken/pork to make a shellfish-forward broth