/**
 * v-for object
 */
export function renderList<T>(
  source: T,
  renderItem: <K extends keyof T>(
    value: T[K],
    key: K,
    index: number
  ) => VNodeChild
): VNodeChild[]

const testObject: { [k: string]: string } = {
key1: 'value2',
key2: 'value2'
}

type k = keyof typeof testObject . the type K is string | number

maybe this is the problem

Thanks for your input, it seems like the types for v-for are declared here:
https://github.com/vuejs/core/blob/3.2/packages/runtime-core/src/helpers/renderList.ts#L41
And we can see that the second argument key gets the type K which is K extends keyof T (with T the type of the v-for source).
So in my case keyof { [key: string]: string } evaluates to string | number.
This was introduced with Typescript 2.9: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-9.html#support-number-and-symbol-named-properties-with-keyof-and-mapped-types

The use of keyof seems logical, however in practice using v-for on an object will always provide the key as a string, so maybe Vue should directly type the key as string?