type ExcludeKey<T extends Record<string,unknown>, U extends keyof T>={
  [K in keyof T as K extends U ? never : K]: T[K]
} 

type MakeKeyOptional<T extends Record<string,unknown>, U extends keyof T>=ExcludeKey<T,U> & {[K in U]?:T[K]}

type A = MakeKeyOptional<{a:1,b:2,c:3},"b"|"c">

const a1:A = {a:1} // ok!
const a2:A = {a:1,b:2} // ok!
const a3:A = {a:1,b:2,c:3} // ok!
const a4:A = {b:2,c:3} // expect error!

type MakeKeyOptional<T extends Record<string, unknown>, U extends keyof T> =
  Omit<T, U> & Partial<Pick<T, U>>;