Adds the label "newsletters" to any Gmail thread that has any label under it (like "newsletters/blogs")

Code.gs

function fixNewsletterLabels() {
  const LABEL_NAME = PropertiesService.getScriptProperties().getProperty('root label') ?? "newsletters"
  let rootLabel = GmailApp.getUserLabelByName(LABEL_NAME)

  // Get the "newsletters" label and all labels under it
  let newsLabelsNames = GmailApp.getUserLabels()
    .map(label => label.getName())
    .filter(labelName => labelName.startsWith(LABEL_NAME + "/"));

  let query = "-label:newsletters ";
  query += newsLabelsNames
    .map(name => `label:${name.replaceAll(/[/ ]/g, '-').toLowerCase()}`)
    .join(" OR ");
  Logger.log(query);

  let threads = GmailApp.search(query);
  Logger.log(`found ${threads.length} emails with labels under #[${LABEL_NAME}/] without #[${LABEL_NAME}]`);

  for (let thread of threads) {
    Logger.log(`Adding #[${LABEL_NAME}] to ${thread.getLastMessageDate().toISOString()} "${thread.getFirstMessageSubject()}"`);
    thread.addLabel(rootLabel)
  }
}