The function here definitely returns something, so its return type shouldn't be Promise<void>.

Here is my suggestion. I didn't look very closely so I might have missed some fields.

type VersioningStatus = {
  "Status": "Enabled" | "Suspended",
  "ExcludeFolders": boolean,
  "ExcludedPrefixes": {"Prefix": string}[]
}

getBucketVersioning(bucketName: string): Promise<"" | VersioningStatus>;

Note that it currently returns an empty string when versioning has never been toggled on. It might be a good idea to return {Status: "Disabled"} instead.