Incorrect (everything tracked):

import Component from '@glimmer/component';
import { tracked } from '@glimmer/tracking';
import { action } from '@ember/object';

class Form extends Component {
  @tracked firstName = ''; // Used in template ✓
  @tracked lastName = ''; // Used in template ✓
  @tracked _formId = Date.now(); // Internal, never rendered ✗
  @tracked _validationCache = new Map(); // Internal state ✗

  @action
  validate() {
    this._validationCache.set('firstName', this.firstName.length > 0);
    // Unnecessary re-render triggered
  }
}