Type narrowing with mapped types. This gist, iterate over nested union types and narrows the type definition for later filtering.

type_narrowing.ts

type Sitemap = {
  pageIdentifier?: 'Page';
  slug?: string | null;
  seo?: { pageIdentifier: 'Seo'; noIndex?: boolean | null } | null;
  content?:
    | { pageIdentifier?: 'AppHomePage' }
    | {
        pageIdentifier: 'CategoryPage';
        blockedInCountry?: boolean | null;
        link?: string | null;
        published?: boolean | null;
        globalCanonical?: { pageIdentifier?: 'Page'; slug?: string | null } | null;
        localizedCanonical?: { pageIdentifier?: 'Page'; slug?: string | null } | null;
      }
    | {
        pageIdentifier: 'ContentPage';
        title: string;
        globalCanonical?: { pageIdentifier?: 'Page'; slug?: string | null } | null;
        localizedCanonical?: { pageIdentifier?: 'Page'; slug?: string | null } | null;
      }
    | { pageIdentifier: 'StartPage' }
    | { pageIdentifier: 'SystemPage' }
    | { pageIdentifier: 'WorldHomePage' }
    | null;
};

type CategoryPage = {
  [Key in keyof Sitemap]: Sitemap[Key] extends Sitemap['content']
    ? Extract<Sitemap['content'], { pageIdentifier: 'CategoryPage' }>
    : Sitemap[Key]
}

type ContentPage = {
  [Key in keyof Sitemap]: Sitemap[Key] extends Sitemap['content']
    ? Extract<Sitemap['content'], { pageIdentifier: 'ContentPage' }>
    : Sitemap[Key]
}

const isCategoryPageOrContentPage = <Page extends CategoryPage | ContentPage>(
  item: Sitemap | null,
): item is Page =>
  item?.content?.pageIdentifier === 'CategoryPage' || item?.content?.pageIdentifier === 'ContentPage'

const item: Sitemap = {}

console.log(isCategoryPageOrContentPage<CategoryPage>(item)) // Narrows type of `item` for `content` field is `pageIdentifier: 'CategoryPage'`
console.log(isCategoryPageOrContentPage<ContentPage>(item)) // Narrows type of `item` for `content` field is `pageIdentifier: 'ContentPage'`
console.log(isCategoryPageOrContentPage<ContentPage | CategoryPage>(item)) // // Narrows type of `item` for `content` field can be either `pageIdentifier: 'CategoryPage'` or `pageIdentifier: 'ContentPage'`

function isContentPageItem(item: Sitemap): false | void {
  //narrows the `content` field type only for  `pageIdentifier: 'ContentPage'`

  if(!isCategoryPageOrContentPage<ContentPage>(item)) { return false }

  console.log(item.content?.title);
}

function isCategoryPageItem(item: Sitemap): false | void {
  //narrows the `content` field type only for  `pageIdentifier: 'CategoryPage'`

  if(!isCategoryPageOrContentPage<CategoryPage>(item)) { return false }

  console.log(item.content?.published);
}

Playground link:

https://www.typescriptlang.org/play?ssl=64&ssc=2&pln=1&pc=1#code/C4TwDgpgBAyglsCBbAhmKBeKBvAUFKMFAcwgEkATCAO2DgDM4IAnAfgC4oByABRIi4BufFADOAGwCuxDmODM41YlAA+UapPHjhBURAD2s7IX6UadRi05cYBoev1lqVAB6yARvv3iIKaqvVNcSgAXwCNLR0oAGN9WnMOEQI1YyJSM1oGJjZrAEEwMAAJfSQIPlIuUKSAvAI6urTyKkzLZmsAYRREYn1mEHKBKPqCd3F9aIBrCAondv1JWj6PLx8-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-BsBgAeWYLihSbLArwQpcV2I2sfz-RtSACeTI34ACAAoRFAkCN3QZkX1wABKThQLEqAEIwICCFA1gADodyfDyP1JZhMAwLBOM0qToVUNQ3M8x8Enc3zHQCoK9IpISROAHtpzMu9EKgkTHncsZiEM64JK0mS5OihTSCvErQtIIzQOs6yoAAemaqAADlw2YfQAHc12raAMIAA1AoaoHoXooCGrzzDGywCwcoa4uDb963-aEhuEuJRDygqivEkL1tIWSkv4aqKv0uqiunRqWrazrmG6vqoAGqBhtG8bJumi65qYBbrim5av04i6KU23KfHy-RCuKw7tIgE7QbO06dLUGqjogeqbqa1q7o6rrev6mC3qHEbpzmr6ZtoX6IAWwF-HcaAIAQO5-KW+1Pw49H4a4MavqBrmkYqTbcHoBZLzgOIxJR8hp2u5BMpnWzxu+PQAgAN30OAKBbAhWuoAnnuAO4pqp4AaYW164nEbsJv8wGOb8johYEEXXPoQyAEJYbW+HEd3AZzoDgz5aQW7jGYCBgEkZh-HoVXoCgggIYgKGYenKLdw8sUfGs4QoLF6gJaln3JIxsg5dMviwGV+PxDVtRNe13W7oNx7CZek3vt3C2dat6gbc++32bYzm2lWsuebdnsPe9g7fbKmXqrhgYseQcOoEj6PY5V+vE6E5Ptt26HQ8z7zYsubZbmmPPcBCIA