How to extend the RPG Maker MZ DataManager to store custom stuff for your plugin.

RMMZ_DataManager_Store.js

  //===========================================================================
  // Extend DataManager to persist specific changes
  //===========================================================================

  const RMMZ_DataManager_makeSaveContents = DataManager.makeSaveContents;
  const RMMZ_DataManager_extractSaveContents = DataManager.extractSaveContents;
  const RMMZ_DataManager_onLoad = DataManager.onLoad;

  DataManager.makeSaveContents = function() {
    const contents = RMMZ_DataManager_makeSaveContents.call(this);
    contents._plugin_store = {};
  }

  DataManager.extractSaveContents = function(contents) {
    RMMZ_DataManager_extractSaveContents.call(this, contents);
    pluginRoot.data = contents._plugin_store;
  }

  DataManager.onLoad = function () {
    RMMZ_DataManager_onLoad.apply(this, arguments);
    console.log(arguments);
  }